Chapter 1. General Information

Table of Contents

1.1. About This Manual
1.2. Conventions Used in This Manual
1.3. Overview of MySQL AB
1.4. Overview of the MySQL Database Management System
1.4.1. History of MySQL
1.4.2. The Main Features of MySQL
1.4.3. MySQL Stability
1.4.4. How Big MySQL Tables Can Be
1.4.5. Year 2000 Compliance
1.5. Overview of the MaxDB Database Management System
1.5.1. What is MaxDB?
1.5.2. History of MaxDB
1.5.3. Features of MaxDB
1.5.4. Licensing and Support
1.5.5. Feature Differences Between MaxDB and MySQL
1.5.6. Interoperability Features Between MaxDB and MySQL
1.5.7. MaxDB-Related Links
1.6. MySQL Development Roadmap
1.6.1. What's New in MySQL 5.0
1.7. MySQL Information Sources
1.7.1. MySQL Mailing Lists
1.7.2. MySQL Community Support on IRC (Internet Relay Chat)
1.7.3. MySQL Community Support at the MySQL Forums
1.8. MySQL Standards Compliance
1.8.1. What Standards MySQL Follows
1.8.2. Selecting SQL Modes
1.8.3. Running MySQL in ANSI Mode
1.8.4. MySQL Extensions to Standard SQL
1.8.5. MySQL Differences from Standard SQL
1.8.6. How MySQL Deals with Constraints

The MySQL® software delivers a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server. MySQL Server is intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software. MySQL is a registered trademark of MySQL AB.

The MySQL software is Dual Licensed. Users can choose to use the MySQL software as an Open Source product under the terms of the GNU General Public License (http://www.fsf.org/licenses/) or can purchase a standard commercial license from MySQL AB. See http://www.mysql.com/company/legal/licensing/ for more information on our licensing policies.

The following list describes some sections of particular interest in this manual:

Important:

Reports of errors (often called “bugs”), as well as questions and comments, should be sent to http://bugs.mysql.com. See Section 1.7.1.3, “How to Report Bugs or Problems”.

If you have found a sensitive security bug in MySQL Server, please let us know immediately by sending an email message to .

1.1. About This Manual

This is the Reference Manual for the MySQL Database System, Version 5.0, up to Release 5.0.16. It is not intended for use with older versions of the MySQL software due to the many functional and other differences between MySQL 5.0 and previous versions. If you are using a Version 4.1 release of the MySQL software, please refer to the MySQL 4.1 Reference Manual, which provides coverage of the 3.22, 3.23, 4.0, and 4.1 series of MySQL software releases. Differences between minor versions of MySQL 5.0 are noted in the present text with reference to release numbers (5.0.x).

Because this manual serves as a reference, it does not provide general instruction on SQL or relational database concepts. It also does not teach you how to use your operating system or command-line interpreter.

The MySQL Database Software is under constant development, and the Reference Manual is updated frequently as well. The most recent version of the manual is available online in searchable form at http://dev.mysql.com/doc/. Other formats also are available, including HTML, PDF, and Windows CHM versions.

The primary document is a set of DocBook XML files. The HTML version and other formats are produced automatically using, among other tools, the DocBook XSL stylesheets.

If you have any suggestions concerning additions or corrections to this manual, please send them to the documentation team at .

This manual was initially written by David Axmark and Michael “Monty” Widenius. It is maintained by the MySQL Documentation Team, consisting of Paul DuBois, Stefan Hinz, Mike Hillyer, and Jon Stephens. For the many other contributors, see Appendix C, Credits.

The copyright to this manual is owned by the Swedish company MySQL AB. MySQL® and the MySQL logo are registered trademarks of MySQL AB. Other trademarks and registered trademarks referred to in this manual are the property of their respective owners, and are used for identification purposes only.

1.2. Conventions Used in This Manual

This manual uses certain typographical conventions:

  • Text in this style is used for SQL statements; database, table, and column names; C and Perl code; and environment variables. Example: “To reload the grant tables, use the FLUSH PRIVILEGES statement”.

    Text in this style is used to indicate input that you type in examples.

  • Text in this style is used to indicate the names of executable programs and scripts, examples being mysql (the MySQL command line client program) and mysqld (the MySQL server executable).

  • Text in this style is used for variable input for which you should substitute a value of your own choosing.

  • Filenames and directory names are written like this: “The global my.cnf file is located in the /etc directory”.

  • Character sequences are written like this: “To specify a wildcard, use the ‘%’ character”.

  • Text in this style is used for emphasis.

  • Text in this style is used in table headings and to convey especially strong emphasis.

When commands are shown that are meant to be executed from within a particular program, the program is indicated by a prompt shown before the command. For example, shell> indicates a command that you execute from your login shell, and mysql> indicates a statement that you execute from the mysql client program:

shell> type a shell command here
mysql> type a mysql statement here

The “shell” is your command interpreter. On Unix, this is typically a program such as sh, csh, or bash. On Windows, the equivalent program is command.com or cmd.exe, typically run in a console window.

When you enter a command or statement shown in an example, do not type the prompt shown in the example.

Database, table, and column names must often be substituted into statements. To indicate that such substitution is necessary, this manual uses db_name, tbl_name, and col_name. For example, you might see a statement like this:

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this:

mysql> SELECT author_name FROM biblio_db.author_list;

SQL keywords are not case sensitive and may be written in uppercase or lowercase. This manual uses uppercase.

In syntax descriptions, square brackets (‘[’ and ‘]’) are used to indicate optional words or clauses. For example, in the following statement, IF EXISTS is optional:

DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (‘|’). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (‘[’ and ‘]’):

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

When one member from a set of choices must be chosen, the alternatives are listed within braces (‘{’ and ‘}’):

{DESCRIBE | DESC} tbl_name [col_name | wild]

An ellipsis (...) indicates the omission of a section of a statement, typically to provide a shorter version of more complex syntax. For example, INSERT ... SELECT is shorthand for the form of INSERT statement that is followed by a SELECT statement.

An ellipsis can also indicate that the preceding syntax element of a statement may be repeated. In the following example, multiple reset_option values may be given, with each of those after the first preceded by commas:

RESET reset_option [,reset_option] ...

Commands for setting shell variables are shown using Bourne shell syntax. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax:

shell> VARNAME=value some_command

If you are using csh or tcsh, you must issue commands somewhat differently. You would execute the sequence just shown like this:

shell> setenv VARNAME value
shell> some_command

1.3. Overview of MySQL AB

MySQL AB is the company of the MySQL founders and main developers. MySQL AB was originally established in Sweden by David Axmark, Allan Larsson, and Michael “Monty” Widenius.

We are dedicated to developing the MySQL database software and promoting it to new users. MySQL AB owns the copyright to the MySQL source code, the MySQL logo and (registered) trademark, and this manual. See Section 1.4, “Overview of the MySQL Database Management System”.

The MySQL core values show our dedication to MySQL and Open Source.

These core values direct how MySQL AB works with the MySQL server software:

  • To be the best and the most widely used database in the world

  • To be available and affordable by all

  • To be easy to use

  • To be continuously improved while remaining fast and safe

  • To be fun to use and improve

  • To be free from bugs

These are the core values of the company MySQL AB and its employees:

  • We subscribe to the Open Source philosophy and support the Open Source community

  • We aim to be good citizens

  • We prefer partners that share our values and mindset

  • We answer email and provide support

  • We are a virtual company, networking with others

  • We work against software patents

The MySQL Web site (http://www.mysql.com/) provides the latest information about MySQL and MySQL AB.

By the way, the “AB” part of the company name is the acronym for the Swedish “aktiebolag,” or “stock company.” It translates to “MySQL, Inc.” In fact, MySQL, Inc. and MySQL GmbH are examples of MySQL AB subsidiaries. They are located in the US and Germany, respectively.

1.4. Overview of the MySQL Database Management System

MySQL, the most popular Open Source SQL database management system, is developed, distributed, and supported by MySQL AB. MySQL AB is a commercial company, founded by the MySQL developers. It is a second generation Open Source company that unites Open Source values and methodology with a successful business model.

The MySQL Web site (http://www.mysql.com/) provides the latest information about MySQL software and MySQL AB.

  • MySQL is a database management system.

    A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL Server. Since computers are very good at handling large amounts of data, database management systems play a central role in computing, as standalone utilities or as parts of other applications.

  • MySQL is a relational database management system.

    A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The SQL part of “MySQL” stands for “Structured Query Language.” SQL is the most common standardized language used to access databases and is defined by the ANSI/ISO SQL Standard. The SQL standard has been evolving since 1986 and several versions exist. In this manual, “SQL-92” refers to the standard released in 1992, “SQL:1999” refers to the standard released in 1999, and “SQL:2003” refers to the current version of the standard. We use the phrase “the SQL standard” to mean the current version of the SQL Standard at any time.

  • MySQL software is Open Source.

    Open Source means that it is possible for anyone to use and modify the software. Anybody can download the MySQL software from the Internet and use it without paying anything. If you wish, you may study the source code and change it to suit your needs. The MySQL software uses the GPL (GNU General Public License), http://www.fsf.org/licenses/, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL code into a commercial application, you can buy a commercially licensed version from us. See the MySQL Licensing Overview for more information (http://www.mysql.com/company/legal/licensing/).

  • The MySQL Database Server is very fast, reliable, and easy to use.

    If that is what you are looking for, you should give it a try. MySQL Server also has a practical set of features developed in close cooperation with our users. You can find a performance comparison of MySQL Server with other database managers on our benchmark page. See Section 7.1.4, “The MySQL Benchmark Suite”.

    MySQL Server was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Although under constant development, MySQL Server today offers a rich and useful set of functions. Its connectivity, speed, and security make MySQL Server highly suited for accessing databases on the Internet.

  • MySQL Server works in client/server or embedded systems.

    The MySQL Database Software is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a wide range of application programming interfaces (APIs).

    We also provide MySQL Server as an embedded multi-threaded library that you can link into your application to get a smaller, faster, easier-to-manage product.

  • A large amount of contributed MySQL software is available.

    It is very likely that your favorite application or language supports the MySQL Database Server.

The official way to pronounce “MySQL” is “My Ess Que Ell” (not “my sequel”), but we don't mind if you pronounce it as “my sequel” or in some other localized way.

1.4.1. History of MySQL

We started out with the intention of using mSQL to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing, we came to the conclusion that mSQL was not fast enough or flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was designed to allow third-party code that was written for use with mSQL to be ported easily for use with MySQL.

The derivation of the name MySQL is not clear. Our base directory and a large number of our libraries and tools have had the prefix “my” for well over 10 years. However, co-founder Monty Widenius's daughter is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.

The name of the MySQL Dolphin (our logo) is “Sakila,” which was chosen by the founders of MySQL AB from a huge list of names suggested by users in our “Name the Dolphin” contest. The winning name was submitted by Ambrose Twebaze, an Open Source software developer from Swaziland, Africa. According to Ambrose, the feminine name Sakila has its roots in SiSwati, the local language of Swaziland. Sakila is also the name of a town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.

1.4.2. The Main Features of MySQL

The following list describes some of the important characteristics of the MySQL Database Software. See also Section 1.6, “MySQL Development Roadmap” for more information about current and upcoming features.

  • Internals and Portability

    • Written in C and C++.

    • Tested with a broad range of different compilers.

    • Works on many different platforms. See Section 2.1.1, “Operating Systems Supported by MySQL”.

    • Uses GNU Automake, Autoconf, and Libtool for portability.

    • APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl are available. See Chapter 22, APIs and Libraries.

    • Fully multi-threaded using kernel threads. It can easily use multiple CPUs if they are available.

    • Provides transactional and non-transactional storage engines.

    • Uses very fast B-tree disk tables (MyISAM) with index compression.

    • Relatively easy to add another storage engine. This is useful if you want to add an SQL interface to an in-house database.

    • A very fast thread-based memory allocation system.

    • Very fast joins using an optimized one-sweep multi-join.

    • In-memory hash tables, which are used as temporary tables.

    • SQL functions are implemented using a highly optimized class library and should be as fast as possible. Usually there is no memory allocation at all after query initialization.

    • The MySQL code is tested with Purify (a commercial memory leakage detector) as well as with Valgrind, a GPL tool (http://developer.kde.org/~sewardj/).

    • The server is available as a separate program for use in a client/server networked environment. It is also available as a library that can be embedded (linked) into standalone applications. Such applications can be used in isolation or in environments where no network is available.

  • Column Types

    • Many column types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes long, FLOAT, DOUBLE, CHAR, VARCHAR, TEXT, BLOB, DATE, TIME, DATETIME, TIMESTAMP, YEAR, SET, ENUM, and OpenGIS spatial types. See Chapter 11, Column Types.

    • Fixed-length and variable-length records.

  • Statements and Functions

    • Full operator and function support in the SELECT and WHERE clauses of queries. For example:

      mysql> SELECT CONCAT(first_name, ' ', last_name)
          -> FROM citizen
          -> WHERE income/dependents > 10000 AND age > 30;
      
    • Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), COUNT(DISTINCT ...), AVG(), STD(), SUM(), MAX(), MIN(), and GROUP_CONCAT()).

    • Support for LEFT OUTER JOIN and RIGHT OUTER JOIN with both standard SQL and ODBC syntax.

    • Support for aliases on tables and columns as required by standard SQL.

    • DELETE, INSERT, REPLACE, and UPDATE return the number of rows that were changed (affected). It is possible to return the number of rows matched instead by setting a flag when connecting to the server.

    • The MySQL-specific SHOW command can be used to retrieve information about databases, database engines, tables, and indexes. The EXPLAIN command can be used to determine how the optimizer resolves a query.

    • Function names do not clash with table or column names. For example, ABS is a valid column name. The only restriction is that for a function call, no spaces are allowed between the function name and the ‘(’ that follows it. See Section 9.6, “Treatment of Reserved Words in MySQL”.

    • You can mix tables from different databases in the same query (as of MySQL 3.22).

  • Security

    • A privilege and password system that is very flexible and secure, and that allows host-based verification. Passwords are secure because all password traffic is encrypted when you connect to a server.

  • Scalability and Limits

    • Handles large databases. We use MySQL Server with databases that contain 50 million records. We also know of users who use MySQL Server with 60,000 tables and about 5,000,000,000 rows.

    • Up to 64 indexes per table are allowed (32 before MySQL 4.1.2). Each index may consist of 1 to 16 columns or parts of columns. The maximum index width is 1000 bytes (500 before MySQL 4.1.2). An index may use a prefix of a column for CHAR, VARCHAR, BLOB, or TEXT column types.

  • Connectivity

    • Clients can connect to the MySQL server using TCP/IP sockets on any platform. On Windows systems in the NT family (NT, 2000, XP, or 2003), clients can connect using named pipes. On Unix systems, clients can connect using Unix domain socket files.

    • In MySQL versions 4.1 and higher, Windows servers also support shared-memory connections if started with the --shared-memory option. Clients can connect through shared memory by using the --protocol=memory option.

    • The Connector/ODBC (MyODBC) interface provides MySQL support for client programs that use ODBC (Open Database Connectivity) connections. For example, you can use MS Access to connect to your MySQL server. Clients can be run on Windows or Unix. MyODBC source is available. All ODBC 2.5 functions are supported, as are many others. See Chapter 23, Connectors.

    • The Connector/J interface provides MySQL support for Java client programs that use JDBC connections. Clients can be run on Windows or Unix. Connector/J source is available. See Chapter 23, Connectors.

  • Localization

    • The server can provide error messages to clients in many languages. See Section 5.10.2, “Setting the Error Message Language”.

    • Full support for several different character sets, including latin1 (cp1252), german, big5, ujis, and more. For example, the Scandinavian characters ‘å’, ‘ä’ and ‘ö’ are allowed in table and column names. Unicode support is available as of MySQL 4.1.

    • All data is saved in the chosen character set. All comparisons for normal string columns are case-insensitive.

    • Sorting is done according to the chosen character set (using Swedish collation by default). It is possible to change this when the MySQL server is started. To see an example of very advanced sorting, look at the Czech sorting code. MySQL Server supports many different character sets that can be specified at compile time and runtime.

  • Clients and Tools

    • The MySQL server has built-in support for SQL statements to check, optimize, and repair tables. These statements are available from the command line through the mysqlcheck client. MySQL also includes myisamchk, a very fast command-line utility for performing these operations on MyISAM tables. See Chapter 5, Database Administration.

    • All MySQL programs can be invoked with the --help or -? options to obtain online assistance.

1.4.3. MySQL Stability

This section addresses the questions, “How stable is MySQL Server?” and, “Can I depend on MySQL Server in this project?” We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing lists, which are very active in identifying problems as well as reporting types of use.

The original code stems back to the early 1980s. It provides a stable code base, and the ISAM table format used by the original storage engine remains backward-compatible. At TcX, the predecessor of MySQL AB, MySQL code has worked in projects since mid-1996, without any problems. When the MySQL Database Software initially was released to a wider public, our new users quickly found some pieces of untested code. Each new release since then has had fewer portability problems, even though each new release has also had many new features.

Each release of the MySQL Server has been usable. Problems have occurred only when users try code from the “gray zones.” Naturally, new users don't know what the gray zones are; this section therefore attempts to document those areas that are currently known. The descriptions mostly deal with Versions 3.23 and later of MySQL Server. All known and reported bugs are fixed in the latest version, with the exception of those listed in the bugs section, which are design-related. See Section A.8, “Known Issues in MySQL”.

The MySQL Server design is multi-layered with independent modules. Some of the newer modules are listed here with an indication of how well-tested each of them is:

  • Replication (Stable)

    Large groups of servers using replication are in production use, with good results. Work on enhanced replication features is continuing in MySQL 5.x.

  • InnoDB tables (Stable)

    The InnoDB transactional storage engine has been stable since version 3.23.49. InnoDB is being used in large, heavy-load production systems.

  • BDB tables (Stable)

    The Berkeley DB code is very stable, but we are still improving the BDB transactional storage engine interface in MySQL Server.

  • Full-text searches (Stable)

    Full-text searching is widely used. Important feature enhancements were added in MySQL 4.0 and 4.1.

  • MyODBC 3.51 (Stable)

    MyODBC 3.51 uses ODBC SDK 3.51 and is in wide production use. Some issues brought up appear to be application-related and independent of the ODBC driver or underlying database server.

1.4.4. How Big MySQL Tables Can Be

MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the MyISAM storage engine in MySQL 3.23, the maximum table size was increased to 65536 terabytes (2567 – 1 bytes). With this larger allowed table size, the maximum effective table size for MySQL databases is usually determined by operating system constraints on file sizes, not by MySQL internal limits.

The InnoDB storage engine maintains InnoDB tables within a tablespace that can be created from several files. This allows a table to exceed the maximum individual file size. The tablespace can include raw disk partitions, which allows extremely large tables. The maximum tablespace size is 64TB.

The following table lists some examples of operating system file-size limits. This is only a rough guide and is not intended to be definitive. For the most up-to-date information, be sure to check the documentation specific to your operating system.

Operating SystemFile-size Limit
Linux 2.2-Intel 32-bit2GB (LFS: 4GB)
Linux 2.4+(using ext3 filesystem) 4TB
Solaris 9/1016TB
NetWare w/NSS filesystem8TB
win32 w/ FAT/FAT322GB/4GB
win32 w/ NTFS2TB (possibly larger)
MacOS X w/ HFS+2TB

On Linux 2.2, you can get MyISAM tables larger than 2GB in size by using the Large File Support (LFS) patch for the ext2 filesystem. On Linux 2.4, patches also exist for ReiserFS to get support for big files (up to 2TB). Most current Linux distributions are based on kernel 2.4 and include all the required LFS patches. With JFS and XFS, petabyte and larger files are possible on Linux. However, the maximum available file size still depends on several factors, one of them being the filesystem used to store MySQL tables.

For a detailed overview about LFS in Linux, have a look at Andreas Jaeger's Large File Support in Linux page at http://www.suse.de/~aj/linux_lfs.html.

Windows users please note: FAT and VFAT (FAT32) are not considered suitable for production use with MySQL. Use NTFS instead.

By default, MySQL creates MyISAM tables with an internal structure that allows a maximum size of about 4GB. You can check the maximum table size for a table with the SHOW TABLE STATUS statement or with myisamchk -dv tbl_name. See Section 13.5.4, “SHOW Syntax”.

If you need a MyISAM table that is larger than 4GB in size (and your operating system supports large files), the CREATE TABLE statement allows AVG_ROW_LENGTH and MAX_ROWS options. See Section 13.1.5, “CREATE TABLE Syntax”. You can also change these options with ALTER TABLE after the table has been created, to increase the table's maximum allowable size. See Section 13.1.2, “ALTER TABLE Syntax”.

Other ways to work around file-size limits for MyISAM tables are as follows:

1.4.5. Year 2000 Compliance

The MySQL Server itself has no problems with Year 2000 (Y2K) compliance:

  • MySQL Server uses Unix time functions that handle dates into the year 2037 for TIMESTAMP values. For DATE and DATETIME values, dates through the year 9999 are accepted.

  • All MySQL date functions are implemented in one source file, sql/time.cc, and are coded very carefully to be year 2000-safe.

  • In MySQL 3.22 and later, the YEAR column type can store the years 0 and 1901 to 2155 in one byte and display them using two or four digits. All two-digit years are considered to be in the range 1970 to 2069, which means that if you store 01 in a YEAR column, MySQL Server treats it as 2001.

The following simple demonstration illustrates that MySQL Server has no problems with DATE or DATETIME values through the year 9999, and no problems with TIMESTAMP values until after the year 2030:

mysql> DROP TABLE IF EXISTS y2k;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE y2k (date DATE,
    ->                   date_time DATETIME,
    ->                   time_stamp TIMESTAMP);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO y2k VALUES
    -> ('1998-12-31','1998-12-31 23:59:59',19981231235959),
    -> ('1999-01-01','1999-01-01 00:00:00',19990101000000),
    -> ('1999-09-09','1999-09-09 23:59:59',19990909235959),
    -> ('2000-01-01','2000-01-01 00:00:00',20000101000000),
    -> ('2000-02-28','2000-02-28 00:00:00',20000228000000),
    -> ('2000-02-29','2000-02-29 00:00:00',20000229000000),
    -> ('2000-03-01','2000-03-01 00:00:00',20000301000000),
    -> ('2000-12-31','2000-12-31 23:59:59',20001231235959),
    -> ('2001-01-01','2001-01-01 00:00:00',20010101000000),
    -> ('2004-12-31','2004-12-31 23:59:59',20041231235959),
    -> ('2005-01-01','2005-01-01 00:00:00',20050101000000),
    -> ('2030-01-01','2030-01-01 00:00:00',20300101000000),
    -> ('2040-01-01','2040-01-01 00:00:00',20400101000000),
    -> ('9999-12-31','9999-12-31 23:59:59',99991231235959);
Query OK, 14 rows affected (0.01 sec)
Records: 14  Duplicates: 0  Warnings: 2

mysql> SELECT * FROM y2k;
+------------+---------------------+----------------+
| date       | date_time           | time_stamp     |
+------------+---------------------+----------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
| 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
| 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
| 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
| 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
| 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
| 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
| 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
| 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
| 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
| 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
| 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
| 2040-01-01 | 2040-01-01 00:00:00 | 00000000000000 |
| 9999-12-31 | 9999-12-31 23:59:59 | 00000000000000 |
+------------+---------------------+----------------+
14 rows in set (0.00 sec)

The final two TIMESTAMP column values are zero because the year values (2040, 9999) exceed the TIMESTAMP maximum. The TIMESTAMP data type, which is used to store the current time, supports values that range from 19700101000000 to 20300101000000 on 32-bit machines (signed value). On 64-bit machines, TIMESTAMP handles values up to 2106 (unsigned value).

Although MySQL Server itself is Y2K-safe, you may run into problems if you use it with applications that are not Y2K-safe. For example, many old applications store or manipulate years using two-digit values (which are ambiguous) rather than four-digit values. This problem may be compounded by applications that use values such as 00 or 99 as “missing” value indicators. Unfortunately, these problems may be difficult to fix because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Thus, even though MySQL Server has no Y2K problems, it is the application's responsibility to provide unambiguous input. See Section 11.3.4, “Y2K Issues and Date Types” for MySQL Server's rules for dealing with ambiguous date input data that contains two-digit year values.

1.5. Overview of the MaxDB Database Management System

MaxDB is a heavy-duty enterprise database. The database management system is SAP-certified.

MaxDB is the new name of a database management system formerly called SAP DB. In 2003 SAP AG and MySQL AB joined a partnership and re-branded the database system to MaxDB. The development of MaxDB has continued since then as it was done before – through the SAP developer team.

MySQL AB co-operates closely with the MaxDB team at SAP around delivering improvements to the MaxDB product. Joint efforts include development of new native drivers to enable more efficient usage of MaxDB in the Open Source community, and improvement of various documentation to expand the MaxDB user base. Also interoperability features between MySQL and MaxDB database are seen as important, and for instance the new MaxDB Synchronization Manager supports data synchronization from MaxDB to MySQL.

The MaxDB database management system does not share a common code-base with the MySQL database management system. The MaxDB and MySQL database management systems are independent products provided by MySQL AB.

MySQL AB offers a complete portfolio of Professional Services for MaxDB.

1.5.1. What is MaxDB?

MaxDB is a ANSI SQL-92 (entry level) compliant relational database management system (RDBMS) from SAP AG, that is delivered by MySQL AB as well. MaxDB fulfills the needs for enterprise usage: safety, scalability high concurrency and performance . It runs on all major operating systems. Over the years it has proven able to run SAP R/3 and terabytes of data in 24x7 operation.

The database development started in 1977 as a research project at the Technical University of Berlin . In the early 80s it became a database product that subsequently was owned by Nixdorf, Siemens Nixdorf, Software AG and today – by SAP AG. Along this line it has been named VDN, Reflex, Supra 2, DDB/4, Entire SQL-DB-Server and ADABAS D. In 1997 SAP took over the software from software AG and renamed it to SAP DB. Since October 2000 SAP DB sources additionally were released as open source under the GNU General Public License (see Appendix J, GNU General Public License).

In 2003 SAP AG and MySQL AB joined a partnership and re-branded the database system to MaxDB.

1.5.2. History of MaxDB

The history of MaxDB goes back to SAP DB, SAP AG's DBMS, i.e. MaxDB is a re-branded and enhanced version of SAP DB. For many years, MaxDB has been used for small, medium, and large installations of the mySAP Business Suite and other demanding SQL applications requiring an enterprise-class DBMS with regard to the number of users, the transactional workload, and the size of the database.

SAP DB was meant to provide an alternative to third-party database systems such as Oracle, Microsoft SQL Server, and DB2 by IBM. In October 2000, SAP AG released SAP DB under the GNU GPL license (see Appendix J, GNU General Public License), thus making it open source software.

Today, MaxDB is used in about 3,500 SAP customer installations worldwide. Moreover, the majority of all DBMS installations on Unix and Linux within SAP’s IT department rely on MaxDB. MaxDB is tuned towards heavy-duty online transaction processing (OLTP) with several thousand users and database sizes ranging from several hundred GB to multiple TB.

In 2003 SAP and MySQL concluded a partnership and development cooperation agreement. As a result, SAP's database system SAP DB has been delivered under the name of MaxDB by MySQL since the release of version 7.5 (November 2003).

Version 7.5 of MaxDB is a direct advancement of the SAP DB 7.4 code base. Therefore, the MaxDB software version 7.5 can be used as a direct upgrade of previous SAP DB versions starting 7.2.04 and higher.

Now, as before, the former SAP DB development team at SAP AG is responsible for developing and supporting MaxDB. MySQL AB co-operates closely with the MaxDB team at SAP around delivering improvements to the MaxDB product, see Section 1.5, “Overview of the MaxDB Database Management System”. Both SAP AG and MySQL AB handle the sale and distribution of MaxDB. The advancement of MaxDB and the MySQL Server leverages synergies that benefit both product lines.

MaxDB is subjected to SAP AG's complete quality assurance process before it is shipped with SAP solutions or provided as a download from the MySQL site.

1.5.3. Features of MaxDB

MaxDB is a heavy-duty, SAP-certified open source database for OLTP and OLAP usage which offers high reliability, availability, scalability and a very comprehensive feature set. It is targeted for large mySAP Business Suite environments and other applications that require maximum enterprise-level database functionality and complements the MySQL database server.

MaxDB operates as a client/server product. It was developed to meet the needs of installations in OLTP and Data Warehouse/OLAP/Decision Support scenarios. Benefits:

  • Easy configuration and administration: GUI-based Installation Manager and Database Manager as single administration tools for DBMS operations

  • Around-the-clock operation, no planned downtimes, no permanent attendance required: Automatic space management, no need for reorganizations

  • Elaborate backup and restore capabilities: Online and incremental backups, recovery wizard to guide you through the recovery scenario

  • Supports large number of users, database sizes in the terabytes, and demanding workloads: Proven reliability, performance, and scalability

  • High availability: Cluster support, standby configuration, hot standby configuration

1.5.4. Licensing and Support

MaxDB can be used under the same licenses available for the other products distributed by MySQL AB. Thus, MaxDB is available under the GNU General Public License, and a commercial license. For more information on licensing, see http://www.mysql.com/company/legal/licensing/.

MySQL AB offers MaxDB technical support to non-SAP customers. The MaxDB support is available on various levels (Basic, Silver and Gold), which expand from unlimited email/web-support to 24x7 phone support for business critical systems.

MySQL AB also offers Licenses and Support for MaxDB when used with SAP Applications, like SAP NetWeaver and mySAP Business Suite. Please contact MySQL AB for more information on licenses and support for your needs.

Consulting and training services are available. MySQL gives classes on MaxDB in regular intervals, see http://www.mysql.com/training/ for a list of classes.

1.5.5. Feature Differences Between MaxDB and MySQL

MaxDB is MySQL AB's SAP-certified database. The MaxDB database server complements the MySQL AB product portfolio. Some MaxDB features are not available on the MySQL database management server and vice versa.

The following list provides a short summary of the main differences between MaxDB and MySQL; it is not complete.

  • MaxDB runs as a client/server system. MySQL can run as a client/server system or as an embedded system.

  • MaxDB might not run on all platforms supported by MySQL.

  • MaxDB uses a proprietary network protocol for client/server communication. MySQL uses either TCP/IP (with or without SSL encryption), sockets (under Unix-like systems), or named pipes (under Windows NT-family systems).

  • MaxDB supports stored procedures. For MySQL, stored procedures are implemented in version 5.0. MaxDB also supports programming of triggers through an SQL extension, which is scheduled for MySQL 5.1. MaxDB contains a debugger for stored procedure languages, can cascade nested triggers, and supports multiple triggers per action and row.

  • MaxDB is distributed with user interfaces that are text-based, graphical, or Web-based. MySQL is distributed with text-based user interfaces only; graphical user interface (MySQL Control Center, MySQL Administrator) are shipped separately from the main distributions. Web-based user interfaces for MySQL are offered by third parties.

  • MaxDB supports a number of programming interfaces that also are supported by MySQL. For developing with MaxDB, the MaxDB ODBC Driver, SQL Database Connectivity (SQLDBC), JDBC Driver, Perl and Python modules and a MaxDB PHP extension, which provides access to the MySQL MaxDB databases using PHP, are available. Third Party Programming Interfaces: Support for OLE DB, ADO, DAO, RDO and .NET through ODBC. MaxDB supports embedded SQL with C/C++.

  • MaxDB includes administrative features that MySQL does not have: job scheduling by time, event, and alert, and sending messages to a database administrator on alert thresholds.

1.5.6. Interoperability Features Between MaxDB and MySQL

MaxDB and MySQL are independent database management servers. The interoperation of the systems is possible in a way that the systems can exchange their data. To exchange data between MaxDB and MySQL you can use the import and export tools of the systems or the MaxDB Synchronization Manager. The import and export tools can be used to transfer data in a in-frequently, manual fashion. The MaxDB Synchronization Manager offers faster, automatic data transfer capabilities.

The MaxDB Loader can be used to export data and object definitions. The Loader can export data using MaxDB internal, binary formats and text formats (CSV). Data exported from MaxDB in text formats can be re-imported to MySQL using the mysqldump database backup program. In order to import MySQL data into MaxDB you can either use mysqldump to create INSERT statements or SELECT ... INTO OUTFILE to create a text file (CSV). Use the MaxDB Loader to load the data files generated by MySQL.

Object definitions can be exchanged between the systems using MaxDB Loader and the MySQL tool mysqldump. As the SQL dialects of both systems differ slightly and MaxDB has features currently not supported by MySQL like SQL constraints, we recommend to hand-tune the definition files. The mysqldump tool offers an option --compatible-name = maxdb to produce output that is compatible to MaxDB to make porting easier.

As part of MaxDB 7.6, the MaxDB Synchronization Manager is released. The Synchronization Manager supports creation of asynchronous replication scenarios between several MaxDB instances. However, interoperability features also are planned, so that the Synchronization Manager supports replication to and from a MySQL server.

In the first release, the Synchronization Manager supports inserting data into MySQL. This means that initially only replication from MaxDB to MySQL is supported. In the course of 2005, exporting of data from a MySQL server to the Synchronization Manager will be added, thus adding support for MySQL to MaxDB replication scenarios.

1.5.7. MaxDB-Related Links

The main page for information about MaxDB is http://www.mysql.com/products/maxdb. The page gives details about the features of the MaxDB database management systems and has pointers to documentation available.

The MySQL Reference Manual does not contain any MaxDB documentation but the introduction given in this section. MaxDB has it's own documentation, called the MaxDB library. The MaxDB library is available on http://dev.mysql.com/doc/maxdb/index.html.

MySQL AB runs a community mailing list on MaxDB, see http://lists.mysql.com/maxdb. The list shows a vivid community discussion. Many of the core developers contribute to it. Product announcements are send to list.

A web forum on MaxDB is on http://forums.mysql.com/. The forum is for questions on MaxDB not related to SAP application.

1.6. MySQL Development Roadmap

This section provides a snapshot of the MySQL development roadmap, including major features implemented in or planned for the various MySQL releases. The following sections provide information for each release series.

The current production release series is MySQL 5.0, which was declared stable for production use as of Version 5.0.15, released in October 2005. The previous production release series was MySQL 4.1, which was declared stable for production use as of Version 4.1.7, released in October 2004. “Production status” means that future 5.0 and 4.1 development is limited only to bugfixes. For the older MySQL 4.0 and 3.23 series, only critical bugfixes are made.

Active MySQL development is currently taking place in the MySQL 5.0 and 5.1 release series, and new features are being added only to the latter.

Before upgrading from one release series to the next, please see the notes at Section 2.10, “Upgrading MySQL”.

The most requested features and the versions in which they were implemented or are scheduled for implementation are summarized in the following table:

FeatureMySQL Series
Foreign keys3.23 (for the InnoDB storage engine)
Unions4.0
Subqueries4.1
R-trees4.1 (for the MyISAM storage engine)
Stored procedures5.0
Views5.0
Cursors5.0
XA transactions5.0
Foreign keys5.1 (implemented in 3.23 for InnoDB)
Triggers5.0 and 5.1
Full outer joins5.1
Constraints5.1 (implemented in 3.23 for InnoDB)
Partitioning5.1
Pluggable Storage Engine API5.1
Row-Based Replication5.1

1.6.1. What's New in MySQL 5.0

The following features are implemented in MySQL 5.0.

  • BIT Data Type: Can be used to store numbers in binary notation. See Section 11.1.1, “Overview of Numeric Types”.

  • Cursors: Elementary support for server-side cursors. For information about using cursors within stored routines, see Section 17.2.11, “Cursors”. For information about using cursors from within the C API, see Section 22.2.7.3, “mysql_stmt_attr_set().

  • Information Schema: The introduction of the INFORMATION_SCHEMA database in MySQL 5.0 provided a standards-compliant means for accessing the MySQL Server's metadata, that is, data about the databases (schemas) on the server and the objects which they contain. See Chapter 20, The INFORMATION_SCHEMA Information Database.

  • Instance Manager: Can be used to start and stop the MySQL Server, even from a remote host. See Section 5.2, “mysqlmanager — The MySQL Instance Manager”.

  • Precision Math: MySQL 5.0 introduced stricter criteria for acceptance or rejection of data, and implemented a new library for fixed-point arithmetic. These contributed to a much higher degree of accuracy for mathematical operations and greater control over invalid values. See Chapter 21, Precision Math.

  • Storage Engines: Storage engines added in MySQL 5.0 include ARCHIVE and FEDERATED. See Section 14.8, “The ARCHIVE Storage Engine” and Section 14.7, “The FEDERATED Storage Engine”.

  • Stored Routines: Support for named stored procedures and stored functions was implemented in MySQL 5.0. See Chapter 17, Stored Procedures and Functions.

  • Strict Mode and Standard Error Handling: MySQL 5.0 added a strict mode where by it follows standard SQL in a number of ways in which it did not previously. Support for standard SQLSTATE error messages was also implemented. See Section 5.3.2, “The Server SQL Mode”.

  • Triggers: MySQL 5.0 added limited support for triggers. See Chapter 18, Triggers and Section 1.8.5.4, “Stored Procedures and Triggers”.

  • VARCHAR Data Type: The maximum effective length of a VARCHAR column was increased to 65,532 bytes, and stripping of trailing whitespace was eliminated. See Section 11.4, “String Types”.

  • Views: MySQL 5.0 added support for named, updateable views. See Chapter 19, Views and Section 1.8.5.6, “Views”.

  • XA Transactions: See Section 13.4.7, “XA Transactions”.

  • Performance enhancements: A number of improvements were made in MySQL 5.0 to improve the speed of certain types of queries and in the handling of certain types. These include:

    • MySQL 5.0 introduces a new “greedy” optimizer which can greatly reduce the time required to arrive at a query execution plan. This is particularly noticeable where several tables are to be joined and no good join keys can otherwise be found. Without the greedy optimizer, the complexity of the search for an execution plan is calculated as N!, where N is the number of tables to be joined. The greedy optimizer reduces this to N!/(D-1)!, where D is the depth of the search. While the greedy optimizer does not guarantee the best possible of all execution plans (this is currently being worked on), it can reduce the time spent arriving at an execution plan for a join involving a great many tables — 30, 40, or more — by a factor of as much as 1,000. This should eliminate most if not all situations where users thought that the optimizer had hung when trying to perform joins across many tables.

    • Use of the Index Merge method to obtain better optimization of AND and OR relations over different keys. (Previously, these were optimized only where both relations in the WHERE clause involved the same key.) This also applies to other one-to-one comparison operators (>, <, so on), including = and the IN operator. This means that MySQL can use multiple indexes in retrieving results for conditions such as WHERE key1 > 4 OR key2 < 7 and even combinations of conditions such as WHERE (key1 > 4 OR key2 < 7) AND (key3 >= 10 OR key4 = 1). See Section 7.2.6, “Index Merge Optimization”.

    • A new equality detector finds and optimizes “hidden” equalities in joins. For example, a WHERE clause such as

      t1.c1=t2c2 AND t2.c2=t3.c3 AND t1.c1 < 5
      

      can be reduced to

      t1.c1=t3.c3 AND t2.c2 < 5 AND t3.c3 < 5
      

      These optimizations can be applied with any combination of AND and OR operators. See Section 7.2.10, “How MySQL Optimizes Nested Joins” and Section 7.2.11, “How MySQL Simplifies Outer Joins”.

    • Optimization of NOT IN and NOT BETWEEN relations, reducing or eliminating table scans for queries making use of them by mean of range analysis. The performance of MySQL with regard to these relations now matches its performance with regard to IN and BETWEEN.

    • The VARCHAR data type as implemented in MySQL 5.0 is more efficient than in previous versions, due to the elimination of the old (and nonstandard) removal of trailing spaces during retrival.

    • The addition of a true BIT column type; this type is much more efficient for storage and retrieval of Boolean values than the workarounds required in MySQL in versions previous to 5.0.

    • Performance Improvements in the InnoDB Storage Engine:

      • New compact storage format which can save up to 20% of the disk space required in previous MySQL/InnoDB versions.

      • Faster recovery from a failed or aborted ALTER TABLE.

      • Faster implementation of TRUNCATE.

      (See Section 14.2, “The InnoDB Storage Engine”.)

    • Performance Improvements in the NDBCluster Storage Engine:

      • Faster handling of queries that use IN and BETWEEN.

      • Condition pushdown: In cases involving the comparison of an unindexed column with a constant, this condition is “pushed down” to the cluster where it is evaluated in all partitions simultaneously, eliminating the need to send non-matching records over the network. This can make such queries 10 to 100 times faster than in MySQL 4.1 Cluster.

        See Section 7.2.1, “EXPLAIN Syntax (Get Information About a SELECT)” for more information.

      (See Chapter 15, MySQL Cluster.)

For those wishing to take a look at the bleeding edge of MySQL development, we make our BitKeeper repository for MySQL publicly available. See Section 2.8.3, “Installing from the Development Source Tree”.

1.7. MySQL Information Sources

1.7.1. MySQL Mailing Lists

This section introduces the MySQL mailing lists and provides guidelines as to how the lists should be used. When you subscribe to a mailing list, you receive all postings to the list as email messages. You can also send your own questions and answers to the list.

1.7.1.1. The MySQL Mailing Lists

To subscribe to or unsubscribe from any of the mailing lists described in this section, visit http://lists.mysql.com/. For most of them, you can select the regular version of the list where you get individual messages, or a digest version where you get one large message per day.

Please do not send messages about subscribing or unsubscribing to any of the mailing lists, because such messages are distributed automatically to thousands of other users.

Your local site may have many subscribers to a MySQL mailing list. If so, the site may have a local mailing list, so that messages sent from lists.mysql.com to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the List-ID: or Delivered-To: headers to identify list messages.

The MySQL mailing lists are as follows:

  • announce

    This list is for announcements of new versions of MySQL and related programs. This is a low-volume list to which all MySQL users should subscribe.

  • mysql

    This is the main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer.

  • bugs

    This list is for people who want to stay informed about issues reported since the last release of MySQL or who want to be actively involved in the process of bug hunting and fixing. See Section 1.7.1.3, “How to Report Bugs or Problems”.

  • internals

    This list is for people who work on the MySQL code. This is also the forum for discussions on MySQL development and for posting patches.

  • mysqldoc

    This list is for people who work on the MySQL documentation: people from MySQL AB, translators, and other community members.

  • benchmarks

    This list is for anyone interested in performance issues. Discussions concentrate on database performance (not limited to MySQL), but also include broader categories such as performance of the kernel, filesystem, disk system, and so on.

  • packagers

    This list is for discussions on packaging and distributing MySQL. This is the forum used by distribution maintainers to exchange ideas on packaging MySQL and on ensuring that MySQL looks and feels as similar as possible on all supported platforms and operating systems.

  • java

    This list is for discussions about the MySQL server and Java. It is mostly used to discuss JDBC drivers, including MySQL Connector/J.

  • win32

    This list is for all topics concerning the MySQL software on Microsoft operating systems, such as Windows 9x, Me, NT, 2000, XP, and 2003.

  • myodbc

    This list is for all topics concerning connecting to the MySQL server with ODBC.

  • gui-tools

    This list is for all topics concerning MySQL GUI tools, including MySQL Administrator and the MySQL Control Center graphical client.

  • cluster

    This list is for discussion of MySQL Cluster.

  • dotnet

    This list is for discussion of the MySQL server and the .NET platform. Mostly related to the MySQL Connector/Net provider.

  • plusplus

    This list is for all topics concerning programming with the C++ API for MySQL.

  • perl

    This list is for all topics concerning the Perl support for MySQL with DBD::mysql.

If you're unable to get an answer to your questions from a MySQL mailing list or forum, one option is to purchase support from MySQL AB. This puts you in direct contact with MySQL developers.

The following table shows some MySQL mailing lists in languages other than English. These lists are not operated by MySQL AB.

1.7.1.2. Asking Questions or Reporting Bugs

Before posting a bug report or question, please do the following:

If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, please follow the guidelines on sending mail to a MySQL mailing list, outlined in the next section, before contacting us.

1.7.1.3. How to Report Bugs or Problems

The normal place to report bugs is http://bugs.mysql.com/, which is the address for our bugs database. This database is public, and can be browsed and searched by anyone. If you log in to the system, you can enter new reports.

Writing a good bug report takes patience, but doing it right the first time saves time both for us and for yourself. A good bug report, containing a full test case for the bug, makes it very likely that we will fix the bug in the next release. This section helps you write your report correctly so that you don't waste your time doing things that may not help us much or at all.

We encourage everyone to use the mysqlbug script to generate a bug report (or a report about any problem). mysqlbug can be found in the scripts directory (source distribution) and in the bin directory under your MySQL installation directory (binary distribution). If you are unable to use mysqlbug (for example, if you are running on Windows), it is still vital that you include all the necessary information noted in this section (most importantly, a description of the operating system and the MySQL version).

The mysqlbug script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message. Please read this section carefully and make sure that all the information described here is included in your report.

Preferably, you should test the problem using the latest production or development version of MySQL Server before posting. Anyone should be able to repeat the bug by just using mysql test < script_file on the included test case or by running the shell or Perl script that is included in the bug report.

All bugs posted in the bugs database at http://bugs.mysql.com/ are corrected or documented in the next MySQL release. If only minor code changes are needed to correct a problem, we may also post a patch that fixes the problem.

If you have found a sensitive security bug in MySQL, you can send email to .

If you have a repeatable bug report, please report it to the bugs database at http://bugs.mysql.com/. Note that even in this case it's good to run the mysqlbug script first to find information about your system. Any bug that we are able to repeat has a high chance of being fixed in the next MySQL release.

To report other problems, you can use one of the MySQL mailing lists.

Remember that it is possible for us to respond to a message containing too much information, but not to one containing too little. People often omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is this: If you are in doubt about stating something, state it. It is faster and less troublesome to write a couple more lines in your report than to wait longer for the answer if we must ask you to provide information that was missing from the initial report.

The most common errors made in bug reports are (a) not including the version number of the MySQL distribution used, and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and version number). This is highly relevant information, and in 99 cases out of 100, the bug report is useless without it. Very often we get questions like, “Why doesn't this work for me?” Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed in newer MySQL versions. Sometimes the error is platform-dependent; in such cases, it is next to impossible for us to fix anything without knowing the operating system and the version number of the platform.

If you compiled MySQL from source, remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether your problem depends on your compiler, we need to know what compiler you use. Note that every compiling problem should be regarded as a bug and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, give a good example of everything you did that led to the problem and describe, in exact detail, the problem itself. The best reports are those that include a full example showing how to reproduce the bug or problem. See Section E.1.6, “Making a Test Case If You Experience Table Corruption”.

If a program produces an error message, it is very important to include the message in your report. If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the lettercase should be observed.) You should never try to reproduce from memory what the error message was; instead, copy and paste the entire message into your report.

If you have a problem with Connector/ODBC (MyODBC), please try to generate a trace file and send it with your report. See Section 23.1.1.9, “How to Report MyODBC Problems or Bugs”.

Please remember that many of the people who read your report do so using an 80-column display. When generating reports or examples using the mysql command-line tool, you should therefore use the --vertical option (or the \G statement terminator) for output that would exceed the available width for such a display (for example, with the EXPLAIN SELECT statement; see the example later in this section).

Please include the following information in your report:

  • The version number of the MySQL distribution you are using (for example, MySQL 4.0.12). You can find out which version you are running by executing mysqladmin version. The mysqladmin program can be found in the bin directory under your MySQL installation directory.

  • The manufacturer and model of the machine on which you experience the problem.

  • The operating system name and version. If you work with Windows, you can usually get the name and version number by double-clicking your My Computer icon and pulling down the “Help/About Windows” menu. For most Unix-like operating systems, you can get this information by executing the command uname -a.

  • Sometimes the amount of memory (real and virtual) is relevant. If in doubt, include these values.

  • If you are using a source distribution of the MySQL software, the name and version number of the compiler used are needed. If you have a binary distribution, the distribution name is needed.

  • If the problem occurs during compilation, include the exact error messages and also a few lines of context around the offending code in the file where the error occurs.

  • If mysqld died, you should also report the query that crashed mysqld. You can usually find this out by running mysqld with query logging enabled, and then looking in the log after mysqld crashes See Section E.1.5, “Using Log Files to Find Cause of Errors in mysqld.

  • If a database table is related to the problem, include the output from mysqldump --no-data db_name tbl_name. This is very easy to do and is a powerful way to get information about any table in a database. The information helps us create a situation matching the one you have experienced.

  • For speed-related bugs or problems with SELECT statements, you should always include the output of EXPLAIN SELECT ..., and at least the number of rows that the SELECT statement produces. You should also include the output from SHOW CREATE TABLE tbl_name for each involved table. The more information you give about your situation, the more likely it is that someone can help you.

    The following is an example of a very good bug report. It should be posted with the mysqlbug script. The example uses the mysql command-line tool. Note the use of the \G statement terminator for statements whose output width would otherwise exceed that of an 80-column display device.

    mysql> SHOW VARIABLES;
    mysql> SHOW COLUMNS FROM ...\G
           <output from SHOW COLUMNS>
    mysql> EXPLAIN SELECT ...\G
           <output from EXPLAIN>
    mysql> FLUSH STATUS;
    mysql> SELECT ...;
           <A short version of the output from SELECT,
           including the time taken to run the query>
    mysql> SHOW STATUS;
           <output from SHOW STATUS>
    
  • If a bug or problem occurs while running mysqld, try to provide an input script that reproduces the anomaly. This script should include any necessary source files. The more closely the script can reproduce your situation, the better. If you can make a reproducible test case, you should post it on http://bugs.mysql.com/ for high-priority treatment.

    If you can't provide a script, you should at least include the output from mysqladmin variables extended-status processlist in your mail to provide some information on how your system is performing.

  • If you can't produce a test case with only a few rows, or if the test table is too big to be mailed to the mailing list (more than 10 rows), you should dump your tables using mysqldump and create a README file that describes your problem.

    Create a compressed archive of your files using tar and gzip or zip, and use FTP to transfer the archive to ftp://ftp.mysql.com/pub/mysql/upload/. Then enter the problem into our bugs database at http://bugs.mysql.com/.

  • If you think that the MySQL server produces a strange result from a query, include not only the result, but also your opinion of what the result should be, and an account describing the basis for your opinion.

  • When giving an example of the problem, it's better to use the variable names, table names, and so on that exist in your actual situation than to come up with new names. The problem could be related to the name of a variable or table. These cases are rare, perhaps, but it is better to be safe than sorry. After all, it should be easier for you to provide an example that uses your actual situation, and it is by all means better for us. In case you have data that you don't want to show to others, you can use FTP to transfer it to ftp://ftp.mysql.com/pub/mysql/upload/. If the information is really top secret and you don't want to show it even to us, then go ahead and provide an example using other names, but please regard this as the last choice.

  • Include all the options given to the relevant programs, if possible. For example, indicate the options that you use when you start the mysqld server as well as the options that you use to run any MySQL client programs. The options to programs such as mysqld and mysql, and to the configure script, are often keys to answers and are very relevant. It is never a bad idea to include them. If you use any modules, such as Perl or PHP, please include the version numbers of those as well.

  • If your question is related to the privilege system, please include the output of mysqlaccess, the output of mysqladmin reload, and all the error messages you get when trying to connect. When you test your privileges, you should first run mysqlaccess. After this, execute mysqladmin reload version and try to connect with the program that gives you trouble. mysqlaccess can be found in the bin directory under your MySQL installation directory.

  • If you have a patch for a bug, do include it. But don't assume that the patch is all we need, or that we can use it, if you don't provide some necessary information such as test cases showing the bug that your patch fixes. We might find problems with your patch or we might not understand it at all; if so, we can't use it.

    If we can't verify exactly what the purpose of the patch is, we won't use it. Test cases help us here. Show that the patch handles all the situations that may occur. If we find a borderline case (even a rare one) where the patch won't work, it may be useless.

  • Guesses about what the bug is, why it occurs, or what it depends on are usually wrong. Even the MySQL team can't guess such things without first using a debugger to determine the real cause of a bug.

  • Indicate in your bug report that you have checked the reference manual and mail archive so that others know you have tried to solve the problem yourself.

  • If you get a parse error, please check your syntax closely. If you can't find something wrong with it, it's extremely likely that your current version of MySQL Server doesn't support the syntax you are using. If you are using the current version and the manual at http://dev.mysql.com/doc/ doesn't cover the syntax you are using, MySQL Server doesn't support your query. In this case, your only options are to implement the syntax yourself or email and ask for an offer to implement it.

    If the manual covers the syntax you are using, but you have an older version of MySQL Server, you should check the MySQL change history to see when the syntax was implemented. In this case, you have the option of upgrading to a newer version of MySQL Server. See Appendix D, MySQL Change History.

  • If your problem is that your data appears corrupt or you get errors when you access a particular table, you should first check and then try to repair your tables with CHECK TABLE and REPAIR TABLE or with myisamchk. See Chapter 5, Database Administration.

    If you are running Windows, please verify the value of lower_case_table_names using the SHOW VARIABLES LIKE 'lower_case_table_names' command.

  • If you often get corrupted tables, you should try to find out when and why this happens. In this case, the error log in the MySQL data directory may contain some information about what happened. (This is the file with the .err suffix in the name.) See Section 5.11.1, “The Error Log”. Please include any relevant information from this file in your bug report. Normally mysqld should never crash a table if nothing killed it in the middle of an update. If you can find the cause of mysqld dying, it's much easier for us to provide you with a fix for the problem. See Section A.1, “How to Determine What Is Causing a Problem”.

  • If possible, download and install the most recent version of MySQL Server and check whether it solves your problem. All versions of the MySQL software are thoroughly tested and should work without problems. We believe in making everything as backward-compatible as possible, and you should be able to switch MySQL versions without difficulty. See Section 2.1.2, “Choosing Which MySQL Distribution to Install”.

If you are a support customer, please cross-post the bug report to for higher-priority treatment, as well as to the appropriate mailing list to see whether someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in MyODBC, see Section 23.1.1.9, “How to Report MyODBC Problems or Bugs”.

For solutions to some common problems, see Appendix A, Problems and Common Errors.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem.

1.7.1.4. Guidelines for Answering Questions on the Mailing List

If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message.

Please don't post mail messages from your browser with HTML mode turned on. Many users don't read mail with a browser.

1.7.2. MySQL Community Support on IRC (Internet Relay Chat)

In addition to the various MySQL mailing lists, you can find experienced community people on IRC (Internet Relay Chat). These are the best networks/channels currently known to us:

  • freenode (see http://www.freenode.net/ for servers)

    • #mysql Primarily MySQL questions, but other database and general SQL questions are welcome. Questions about PHP, Perl or C in combination with MySQL are also common.

If you are looking for IRC client software to connect to an IRC network, take a look at xChat (http://www.xchat.org/). X-Chat (GPL licensed) is available for Unix as well as for Windows platforms (a free Windows build of X-Chat is available at http://www.silverex.org/download/).

1.7.3. MySQL Community Support at the MySQL Forums

The latest community support resource are the forums at http://forums.mysql.com.

There are a variety of forums available, grouped in the following general categories:

  • Migration

  • MySQL Usage

  • MySQL Connectors

  • Programming Languages

  • Tools

  • 3rd-Party Applications

  • Storage Engines

  • MySQL Technology

  • SQL Standards

  • Business

1.8. MySQL Standards Compliance

This section describes how MySQL relates to the ANSI/ISO SQL standards. MySQL Server has many extensions to the SQL standard, and here you can find out what they are and how to use them. You can also find information about functionality missing from MySQL Server, and how to work around some differences.

The SQL standard has been evolving since 1986 and several versions exist. In this manual, “SQL-92” refers to the standard released in 1992, “SQL:1999” refers to the standard released in 1999, and “SQL:2003” refers to the current version of the standard. We use the phrase “the SQL standard” to mean the current version of the SQL Standard at any time.

Our goal is to not restrict MySQL Server usability for any usage without a very good reason for doing so. Even if we don't have the resources to perform development for every possible use, we are always willing to help and offer suggestions to people who are trying to use MySQL Server in new territories.

One of our main goals with the product is to continue to work toward compliance with the SQL standard, but without sacrificing speed or reliability. We are not afraid to add extensions to SQL or support for non-SQL features if this greatly increases the usability of MySQL Server for a large segment of our user base. The HANDLER interface in MySQL Server 4.0 is an example of this strategy. See Section 13.2.3, “HANDLER Syntax”.

We continue to support transactional and non-transactional databases to satisfy both mission-critical 24/7 usage and heavy Web or logging usage.

MySQL Server was originally designed to work with medium size databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-size databases, but the code can also be compiled in a reduced version suitable for hand-held and embedded devices. The compact design of the MySQL server makes development in both directions possible without any conflicts in the source tree.

Currently, we are not targeting realtime support, although MySQL replication capabilities offer significant functionality.

Database cluster support exists through third-party clustering solutions as well as the integration of our acquired NDB Cluster technology, available from version 4.1.2. See Chapter 15, MySQL Cluster.

We are also looking at providing XML support in the database server.

1.8.1. What Standards MySQL Follows

We are aiming toward supporting the full ANSI/ISO SQL standard, but without making concessions to speed and quality of the code.

ODBC levels 0-3.51.

1.8.2. Selecting SQL Modes

The MySQL server can operate in different SQL modes, and can apply these modes differentially for different clients. This allows an application to tailor server operation to its own requirements.

Modes define what SQL syntax MySQL should support and what kind of validation checks it should perform on the data. This makes it easier to use MySQL in a lot of different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting mysqld with the --sql-mode="modes" option. Beginning with MySQL 4.1, you can also change the mode after startup time by setting the sql_mode variable with a SET [SESSION|GLOBAL] sql_mode='modes' statement.

For more information on setting the server mode, see Section 5.3.2, “The Server SQL Mode”.

1.8.3. Running MySQL in ANSI Mode

You can tell mysqld to use the ANSI mode with the --ansi startup option. See Section 5.3.1, “mysqld Command-Line Options”.

Running the server in ANSI mode is the same as starting it with these options (specify the --sql_mode value on a single line):

--transaction-isolation=SERIALIZABLE
--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE

In MySQL 4.1, you can achieve the same effect with these two statements (specify the sql_mode value on a single line):

SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET GLOBAL sql_mode = 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE';

See Section 1.8.2, “Selecting SQL Modes”.

In MySQL 4.1.1, the sql_mode options shown can be also be set with this statement:

SET GLOBAL sql_mode='ansi';

In this case, the value of the sql_mode variable is set to all options that are relevant for ANSI mode. You can check the result like this:

mysql> SET GLOBAL sql_mode='ansi';
mysql> SELECT @@global.sql_mode;
        -> 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
            IGNORE_SPACE,ANSI';

1.8.4. MySQL Extensions to Standard SQL

The MySQL Server includes some extensions that you probably won't find in other SQL DBMSs. Be warned that if you use them, your code won't be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form /*! ... */. In this case, MySQL Server parses and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example:

SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...

If you add a version number after the ‘!’ character, the syntax within the comment is executed only if the MySQL version is equal to or newer than the specified version number:

CREATE /*!32302 TEMPORARY */ TABLE t (a INT);

This means that if you have Version 3.23.02 or newer, MySQL Server uses the TEMPORARY keyword.

The following descriptions list MySQL extensions, organized by category.

  • Organization of data on disk

    MySQL Server maps each database to a directory under the MySQL data directory, and tables within a database to filenames in the database directory. This has a few implications:

    • Database names and table names are case sensitive in MySQL Server on operating systems that have case-sensitive filenames (such as most Unix systems). See Section 9.2.2, “Identifier Case Sensitivity”.

    • You can use standard system commands to back up, rename, move, delete, and copy tables that are managed by the MyISAM or ISAM storage engines. For example, to rename a MyISAM table, rename the .MYD, .MYI, and .frm files to which the table corresponds.

    Database, table, index, column, or alias names may begin with a digit (but may not consist solely of digits).

  • General language syntax

    • Strings may be enclosed by either ‘"’ or ‘'’, not just by ‘'’.

    • Use of ‘\’ as an escape character in strings.

    • In SQL statements, you can access tables from different databases with the db_name.tbl_name syntax. Some SQL servers provide the same functionality but call this User space. MySQL Server doesn't support tablespaces such as used in statements like this: CREATE TABLE ralph.my_table...IN my_tablespace.

  • SQL statement syntax

  • Column types

    • The column types MEDIUMINT, SET, ENUM, and the different BLOB and TEXT types.

    • The column attributes AUTO_INCREMENT, BINARY, NULL, UNSIGNED, and ZEROFILL.

  • Functions and operators

    • To make it easier for users who come from other SQL environments, MySQL Server supports aliases for many functions. For example, all string functions support both standard SQL syntax and ODBC syntax.

    • MySQL Server understands the || and && operators to mean logical OR and AND, as in the C programming language. In MySQL Server, || and OR are synonyms, as are && and AND. Because of this nice syntax, MySQL Server doesn't support the standard SQL || operator for string concatenation; use CONCAT() instead. Because CONCAT() takes any number of arguments, it's easy to convert use of the || operator to MySQL Server.

    • Use of COUNT(DISTINCT list) where list has more than one element.

    • All string comparisons are case-insensitive by default, with sort ordering determined by the current character set (cp1252 Latin1 by default). If you don't like this, you should declare your columns with the BINARY attribute or use the BINARY cast, which causes comparisons to be done using the underlying character code values rather then a lexical ordering.

    • The % operator is a synonym for MOD(). That is, N % M is equivalent to MOD(N,M). % is supported for C programmers and for compatibility with PostgreSQL.

    • The =, <>, <=,<, >=,>, <<, >>, <=>, AND, OR, or LIKE operators may be used in column comparisons to the left of the FROM in SELECT statements. For example:

      mysql> SELECT col1=1 AND col2=2 FROM tbl_name;
      
    • The LAST_INSERT_ID() function that returns the most recent AUTO_INCREMENT value. See Section 12.9.3, “Information Functions”.

    • LIKE is allowed on numeric columns.

    • The REGEXP and NOT REGEXP extended regular expression operators.

    • CONCAT() or CHAR() with one argument or more than two arguments. (In MySQL Server, these functions can take any number of arguments.)

    • The BIT_COUNT(), CASE, ELT(), FROM_DAYS(), FORMAT(), IF(), PASSWORD(), ENCRYPT(), MD5(), ENCODE(), DECODE(), PERIOD_ADD(), PERIOD_DIFF(), TO_DAYS(), and WEEKDAY() functions.

    • Use of TRIM() to trim substrings. Standard SQL supports removal of single characters only.

    • The GROUP BY functions STD(), BIT_OR(), BIT_AND(), BIT_XOR(), and GROUP_CONCAT(). See Section 12.10, “Functions and Modifiers for Use with GROUP BY Clauses”.

1.8.5. MySQL Differences from Standard SQL

We try to make MySQL Server follow the ANSI SQL standard and the ODBC SQL standard, but MySQL Server performs operations differently in some cases:

1.8.5.1. Subqueries

MySQL 4.1 supports subqueries and derived tables. A “subquery” is a SELECT statement nested within another statement. A “derived table” (an unnamed view) is a subquery in the FROM clause of another statement. See Section 13.2.8, “Subquery Syntax”.

For MySQL versions older than 4.1, most subqueries can be rewritten using joins or other methods. See Section 13.2.8.11, “Rewriting Subqueries as Joins for Earlier MySQL Versions” for examples that show how to do this.

1.8.5.2. SELECT INTO TABLE

MySQL Server doesn't support the Sybase SQL extension: SELECT ... INTO TABLE .... Instead, MySQL Server supports the standard SQL syntax INSERT INTO ... SELECT ..., which is basically the same thing. See Section 13.2.4.1, “INSERT ... SELECT Syntax”.

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

Alternatively, you can use SELECT INTO OUTFILE ... or CREATE TABLE ... SELECT.

From version 5.0, MySQL supports SELECT ... INTO with user variables. The same syntax may also be used inside stored procedures using cursors and local variables. See Section 17.2.9.3, “SELECT ... INTO Statement”.

1.8.5.3. Transactions and Atomic Operations

MySQL Server (version 3.23-max and all versions 4.0 and above) supports transactions with the InnoDB and BDB transactional storage engines. InnoDB provides full ACID compliance. See Chapter 14, Storage Engines and Table Types.

The other non-transactional storage engines in MySQL Server (such as MyISAM) follow a different paradigm for data integrity called “atomic operations.” In transactional terms, MyISAM tables effectively always operate in AUTOCOMMIT=1 mode. Atomic operations often offer comparable integrity with higher performance.

With MySQL Server supporting both paradigms, you can decide whether your applications are best served by the speed of atomic operations or the use of transactional features. This choice can be made on a per-table basis.

As noted, the trade-off for transactional versus non-transactional table types lies mostly in performance. Transactional tables have significantly higher memory and diskspace requirements, and more CPU overhead. On the other hand, transactional table types such as InnoDB also offer many significant features. MySQL Server's modular design allows the concurrent use of different storage engines to suit different requirements and deliver optimum performance in all situations.

But how do you use the features of MySQL Server to maintain rigorous integrity even with the non-transactional MyISAM tables, and how do these features compare with the transactional table types?

  1. If your applications are written in a way that is dependent on being able to call ROLLBACK rather than COMMIT in critical situations, transactions are more convenient. Transactions also ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved.

    If you use non-transactional tables, MySQL Server in almost all cases allows you to resolve potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such an inconsistency occurs. Note that just by using the MySQL log or even adding one extra log, you can normally fix tables perfectly with no data integrity loss.

  2. More often than not, critical transactional updates can be rewritten to be atomic. Generally speaking, all integrity problems that transactions solve can be done with LOCK TABLES or atomic updates, ensuring that there are no automatic aborts from the server, which is a common problem with transactional database systems.

  3. To be safe with MySQL Server, whether or not using transactional tables, you only need to have backups and have binary logging turned on. With this you can recover from any situation that you could with any other transactional database system. It is always good to have backups, regardless of which database system you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that non-transactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.

In situations where integrity is of highest importance, MySQL Server offers transaction-level reliability and integrity even for non-transactional tables. If you lock tables with LOCK TABLES, all updates stall until integrity checks are made. If you obtain a READ LOCAL lock (as opposed to a write lock) for a table that allows concurrent inserts at the end of the table, reads are allowed, as are inserts by other clients. The newly inserted records are not be seen by the client that has the read lock until it releases the lock. With INSERT DELAYED, you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. See Section 13.2.4.2, “INSERT DELAYED Syntax”.

Atomic,” in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there can never be an automatic rollback (which can happen with transactional tables if you are not very careful). MySQL Server also guarantees that there are no dirty reads.

Following are some techniques for working with non-transactional tables:

  • Loops that need transactions normally can be coded with the help of LOCK TABLES, and you don't need cursors to update records on the fly.

  • To avoid using ROLLBACK, you can use the following strategy:

    1. Use LOCK TABLES to lock all the tables you want to access.

    2. Test the conditions that must be true before performing the update.

    3. Update if everything is okay.

    4. Use UNLOCK TABLES to release your locks.

    This is usually a much faster method than using transactions with possible rollbacks, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In this case, all locks are released but some of the updates may not have been executed.

  • You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:

    • Modify columns relative to their current value.

    • Update only those columns that actually have changed.

    For example, when we are doing updates to some customer information, we update only the customer data that has changed and test only that none of the changed data, or data that depends on the changed data, has changed compared to the original row. The test for changed data is done with the WHERE clause in the UPDATE statement. If the record wasn't updated, we give the client a message: “Some of the data you have changed has been changed by another user.” Then we show the old row versus the new row in a window so that the user can decide which version of the customer record to use.

    This gives us something that is similar to column locking but is actually even better because we only update some of the columns, using values that are relative to their current values. This means that typical UPDATE statements look something like these:

    UPDATE tablename SET pay_back=pay_back+125;
    
    UPDATE customer
      SET
        customer_date='current_date',
        address='new address',
        phone='new phone',
        money_owed_to_us=money_owed_to_us-125
      WHERE
        customer_id=id AND address='old address' AND phone='old phone';
    

    This is very efficient and works even if another client has changed the values in the pay_back or money_owed_to_us columns.

  • In many cases, users have wanted LOCK TABLES and/or ROLLBACK for the purpose of managing unique identifiers. This can be handled much more efficiently without locking or rolling back by using an AUTO_INCREMENT column and either the LAST_INSERT_ID() SQL function or the mysql_insert_id() C API function. See Section 12.9.3, “Information Functions”. See Section 22.2.3.36, “mysql_insert_id().

    You can generally code around the need for row-level locking. Some situations really do need it, and InnoDB tables support row-level locking. With MyISAM tables, you can use a flag column in the table and do something like the following:

    UPDATE tbl_name SET row_flag=1 WHERE id=ID;
    

    MySQL returns 1 for the number of affected rows if the row was found and row_flag wasn't 1 in the original row.

    You can think of it as though MySQL Server changed the preceding query to:

    UPDATE tbl_name SET row_flag=1 WHERE id=ID AND row_flag <> 1;
    

1.8.5.4. Stored Procedures and Triggers

Stored procedures are implemented in MySQL version 5.0. See Chapter 17, Stored Procedures and Functions.

Basic triggers functionality is implemented in MySQL beginning with version 5.0.2, with further development planned in MySQL 5.1. See Chapter 18, Triggers.

1.8.5.5. Foreign Keys

In MySQL Server 3.23.44 and up, the InnoDB storage engine supports checking of foreign key constraints, including CASCADE, ON DELETE, and ON UPDATE. See Section 14.2.6.4, “FOREIGN KEY Constraints”.

For storage engines other than InnoDB, MySQL Server parses the FOREIGN KEY syntax in CREATE TABLE statements, but does not use or store it. In the future, the implementation will be extended to store this information in the table specification file so that it may be retrieved by mysqldump and ODBC. At a later stage, foreign key constraints will be implemented for MyISAM tables as well.

Foreign key enforcement offers several benefits to database developers:

  • Assuming proper design of the relationships, foreign key constraints make it more difficult for a programmer to introduce an inconsistency into the database.

  • Centralized checking of constraints by the database server makes it unnecessary to perform these checks on the application side. This eliminates the possibility that different applications may not all check the constraints in the same way.

  • Using cascading updates and deletes can simplify the application code.

  • Properly designed foreign key rules aid in documenting relationships between tables.

Do keep in mind that these benefits come at the cost of additional overhead for the database server to perform the necessary checks. Additional checking by the server affects performance, which for some applications may be sufficiently undesirable as to be avoided if possible. (Some major commercial applications have coded the foreign-key logic at the application level for this reason.)

MySQL gives database developers the choice of which approach to use. If you don't need foreign keys and want to avoid the overhead associated with enforcing referential integrity, you can choose another table type instead, such as MyISAM. (For example, the MyISAM storage engine offers very fast performance for applications that perform only INSERT and SELECT operations, because the inserts can be performed concurrently with retrievals. See Section 7.3.2, “Table Locking Issues”.)

If you choose not to take advantage of referential integrity checks, keep the following considerations in mind:

  • In the absence of server-side foreign key relationship checking, the application itself must handle relationship issues. For example, it must take care to insert rows into tables in the proper order, and to avoid creating orphaned child records. It must also be able to recover from errors that occur in the middle of multiple-record insert operations.

  • If ON DELETE is the only referential integrity capability an application needs, note that as of MySQL Server 4.0, you can use multiple-table DELETE statements to delete rows from many tables with a single statement. See Section 13.2.1, “DELETE Syntax”.

  • A workaround for the lack of ON DELETE is to add the appropriate DELETE statement to your application when you delete records from a table that has a foreign key. In practice, this is often as quick as using foreign keys, and is more portable.

Be aware that the use of foreign keys can in some instances lead to problems:

  • Foreign key support addresses many referential integrity issues, but it is still necessary to design key relationships carefully to avoid circular rules or incorrect combinations of cascading deletes.

  • It is not uncommon for a DBA to create a topology of relationships that makes it difficult to restore individual tables from a backup. (MySQL alleviates this difficulty by allowing you to temporarily disable foreign key checks when reloading a table that depends on other tables. See Section 14.2.6.4, “FOREIGN KEY Constraints”. As of MySQL 4.1.1, mysqldump generates dump files that take advantage of this capability automatically when reloaded.)

Note that foreign keys in SQL are used to check and enforce referential integrity, not to join tables. If you want to get results from multiple tables from a SELECT statement, you do this by performing a join between them:

SELECT * FROM t1, t2 WHERE t1.id = t2.id;

See Section 13.2.7.1, “JOIN Syntax”. See Section 3.6.6, “Using Foreign Keys”.

The FOREIGN KEY syntax without ON DELETE ... is often used by ODBC applications to produce automatic WHERE clauses.

1.8.5.6. Views

Views (including updatable views) are implemented in the 5.0 version of MySQL Server. Views are available in binary releases from 5.0.1 and up. See Chapter 19, Views.

Views are useful for allowing users to access a set of relations (tables) as if it were a single table, and limiting their access to just that. Views can also be used to restrict access to rows (a subset of a particular table). For access control to columns, you can also use the sophisticated privilege system in MySQL Server. See Section 5.7, “The MySQL Access Privilege System”.

In designing an implementation of views, our ambitious goal, as much as is possible within the confines of SQL, has been full compliance with “Codd's Rule #6” for relational database systems: “All views that are theoretically updatable, should in practice also be updatable.

1.8.5.7. '--' as the Start of a Comment

Some other SQL databases use ‘--’ to start comments. MySQL Server uses ‘#’ as the start comment character. You can also use the C comment style /* this is a comment */ with MySQL Server. See Section 9.5, “Comment Syntax”.

MySQL Server 3.23.3 and above support the ‘--’ comment style, provided the comment is followed by a space (or by a control character such as a newline). The requirement for a space is to prevent problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!:

UPDATE account SET credit=credit-!payment!

Think about what happens if the value of payment is a negative value such as -1:

UPDATE account SET credit=credit--1

credit--1 is a legal expression in SQL, but if -- is interpreted as the start of a comment, part of the expression is discarded. The result is a statement that has a completely different meaning than intended:

UPDATE account SET credit=credit

The statement produces no change in value at all! This illustrates that allowing comments to start with ‘--’ can have serious consequences.

Using our implementation of this method of commenting in MySQL Server 3.23.3 and up, credit--1 is actually safe.

Another safe feature is that the mysql command-line client removes all lines that start with ‘--’.

The following information is relevant only if you are running a MySQL version earlier than 3.23.3:

If you have an SQL program in a text file that contains ‘--’ comments, you should use the replace utility as follows to convert the comments to use ‘#’ characters:

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql db_name

instead of the usual:

shell> mysql db_name < text-file-with-funny-comments.sql

You can also edit the command file “in place” to change the ‘--’ comments to ‘#’ comments:

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command:

shell> replace " #" " --" -- text-file-with-funny-comments.sql

1.8.6. How MySQL Deals with Constraints

MySQL allows you to work both with transactional tables that allow rollback and with non-transactional tables that do not. Because of this, constraint handling is a bit different in MySQL than in other DBMSs. We must handle the case when you have inserted or updated a lot of rows in a non-transactional table for which changes cannot be rolled back when an error occurs.

The basic philosophy is that MySQL Server tries to produce an error for anything that it can detect while parsing a statement to be executed, and tries to recover from any errors that occur while executing the statement. We do this in most cases, but not yet for all.

The options MySQL has when an error occurs are to stop the statement in the middle or to recover as well as possible from the problem and continue. By default, the server follows the latter course. This means, for example, that the server may coerce illegal values to the closest legal values.

Beginning with MySQL 5.0.2, several SQL mode options are available to provide greater control over how accepting to be of bad data values and whether to continue executing a statement or abort it when errors occur. Using these options, you can configure MySQL Server to act in a more traditional fashion that is like other DBMSs that reject improper input. The SQL mode can be set at runtime, which enables individual clients to select the behavior most appropriate for their requirements. See Section 5.3.2, “The Server SQL Mode”.

The following sections describe what happens for the different types of constraints.

1.8.6.1. PRIMARY KEY and UNIQUE Index Constraints

Normally, an error occurs when you try to INSERT or UPDATE a row that causes a primary key, unique key, or foreign key violation. If you are using a transactional storage engine such as InnoDB, MySQL automatically rolls back the statement. If you are using a non-transactional storage engine, MySQL stops processing the statement at the row for which the error occurred and leaves any remaining rows unprocessed.

If you wish to ignore such key violations, MySQL supports an IGNORE keyword for INSERT and UPDATE. In this case, MySQL ignores any key violations and continues processing with the next row. See Section 13.2.4, “INSERT Syntax”. See Section 13.2.10, “UPDATE Syntax”.

You can get information about the number of rows actually inserted or updated with the mysql_info() C API function. See Section 22.2.3.34, “mysql_info(). In MySQL 4.1 and up, you also can use the SHOW WARNINGS statement. See Section 13.5.4.22, “SHOW WARNINGS Syntax”.

For the moment, only InnoDB tables support foreign keys. See Section 14.2.6.4, “FOREIGN KEY Constraints”. Foreign key support in MyISAM tables is scheduled for implementation in MySQL 5.1.

1.8.6.2. Constraints on Invalid Data

Before MySQL 5.0.2, MySQL is forgiving of illegal or improper data values and coerces them to legal values for data entry. In MySQL 5.0.2 and up, that remains the default behavior, but you can select more traditional treatment of bad values such that the server rejects them and aborts the statement in which they occur. This section describes the default (forgiving) behavior of MySQL, as well as the newer strict SQL mode and how it differs.

The following holds true when you are not using strict mode. If you insert an “incorrect” value into a column, such as a NULL into a NOT NULL column or a too-large numeric value into a numeric column, MySQL sets the column to the “best possible value” instead of producing an error:

  • If you try to store an out of range value in a numeric column, MySQL Server instead stores zero, the smallest possible value, or the largest possible value in the column.

  • For strings, MySQL stores either the empty string or as much of the string as can be stored in the column.

  • If you try to store a string that doesn't start with a number into a numeric column, MySQL Server stores 0.

  • MySQL allows you to store certain incorrect date values into DATE and DATETIME columns (such as '2000-02-31' or '2000-02-00'). The idea is that it's not the job of the SQL server to validate dates. If MySQL can store a date value and retrieve exactly the same value, MySQL stores it as given. If the date is totally wrong (outside the server's ability to store it), the special date value '0000-00-00' is stored in the column instead.

  • If you try to store NULL into a column that doesn't take NULL values, an error occurs for single-row INSERT statements. For multiple-row INSERT statements or for INSERT INTO ... SELECT statements, MySQL Server stores the implicit default value for the column data type. In general, this is 0 for numeric types, the empty string ('') for string types, and the “zero” value for date and time types. Implicit default values are discussed in Section 13.1.5, “CREATE TABLE Syntax”.

  • If an INSERT statement specifies no value for a column, MySQL inserts its default value if the column definition includes an explicit DEFAULT clause. If the definition has no such DEFAULT clause, MySQL inserts the implicit default value for the column data type.

The reason for the preceding rules is that we can't check these conditions until the statement has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the storage engine may not support rollback. The option of terminating the statement is not that good; in this case, the update would be “half done,” which is probably the worst possible scenario. In this case, it's better to “do the best you can” and then continue as if nothing happened.

In MySQL 5.0.2 and up, you can select stricter treatment of input values by using the STRICT_TRANS_TABLES or STRICT_ALL_TABLES SQL modes. See Section 5.3.2, “The Server SQL Mode”.

STRICT_TRANS_TABLES works like this:

  • For transactional storage engines, bad data values occurring anywhere in the statement cause the statement to abort and roll back.

  • For non-transactional storage engines, the statement aborts if the error occurs in the first row to be inserted or updated. (In this case, the statement can be regarded to leave the table unchanged, just as for a transactional table.) Errors in rows after the first do not abort the statement. Instead, bad data values are adjusted and result in warnings rather than errors. In other words, with STRICT_TRANS_TABLES, a wrong value causes MySQL to roll back, if it can, all updates done so far.

For stricter checking, enable STRICT_ALL_TABLES. This is the same as STRICT_TRANS_TABLES except that for non-transactional storage engines, errors abort the statement even for bad data in rows following the first row. This means that if an error occurs partway through a multiple-row insert or update for a non-transactional table, a partial update results. Earlier rows are inserted or updated, but those from the point of the error on are not. To avoid this for non-transactional tables, either use single-row statements or else use STRICT_TRANS_TABLES if conversion warnings rather than errors are acceptable. To avoid problems in the first place, do not use MySQL to check column content. It is safest (and often faster) to let the application ensure that it passes only legal values to the database.

With either of the strict mode options, you can cause errors to be treated as warnings by using INSERT IGNORE or UPDATE IGNORE rather than INSERT or UPDATE without IGNORE.

1.8.6.3. ENUM and SET Constraints

ENUM and SET columns provide an efficient way to define columns that can contain only a given set of values. However, before MySQL 5.0.2, ENUM and SET are not real constraints. This is for the same reasons that NOT NULL is not honored. See Section 1.8.6.2, “Constraints on Invalid Data”.

ENUM columns always have a default value. If you don't specify a default value, then it is NULL for columns that can have NULL, otherwise the first enumeration value is used as the default value.

If you insert an incorrect value into an ENUM column or if you force a value into an ENUM column with IGNORE, it is set to the reserved enumeration value of 0, which is displayed as an empty string in string context. See Section 11.4.4, “The ENUM Type”.

If you insert an incorrect value into a SET column, the incorrect value is ignored. For example, if the column can contain the values 'a', 'b', and 'c', an attempt to assign 'a,x,b,y' results in a value of 'a,b'. See Section 11.4.5, “The SET Type”.

As of MySQL 5.0.2, you can configure the server to use strict SQL mode. See Section 5.3.2, “The Server SQL Mode”. When strict mode is enabled, the definition of a ENUM or SET column does act as a constraint on values entered into the column. An error occurs for values that do not satisfy these conditions:

  • An ENUM value must be one of those listed in the column definition, or the internal numeric equivalent thereof. The value cannot be the error value (that is, 0 or the empty string). For a column defined as ENUM('a','b','c'), values such as '', 'd', and 'ax' are illegal and are rejected.

  • A SET value must be the empty string or a value consisting of one or more of the values listed in the column definition separated by commas. For a column defined as SET('a','b','c'), values such as 'd', and 'a,b,c,d' are illegal and are rejected.

Errors for invalid values can be suppressed in strict mode if you use INSERT IGNORE or UPDATE IGNORE. In this case, a warning is generated rather than an error. For ENUM, the value is inserted as the error member (0). For SET, the value is inserted as given except that any invalid substrings are deleted. For example, 'a,x,b,y' results in a value of 'a,b', as described earlier.