Chapter 1. General Information
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## Chapter 1. General Information

End of Product LifecycleActive development and support for MySQL database server versions 3.23, 4.0, and 4.1 has ended. However, for MySQL 4.0 and 4.1, there is still extended support available. For details, see http://www.mysql.com/company/legal/lifecycle/#calendar. According to the MySQL Lifecycle Policy (see http://www.mysql.com/company/legal/lifecycle/#policy), only Security and Severity Level 1 issues will still be fixed for MySQL 4.0 and 4.1. Please consider upgrading to a recent version (MySQL 5.0 or 5.1).

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:

- For a discussion about the capabilities of the MySQL Database Server, see Section 1.4.3, “The Main Features of MySQL”.

- For installation instructions, see Chapter 2, Installing and Upgrading MySQL. For information about upgrading MySQL, see Section 2.11, “Upgrading MySQL”.

- For a tutorial introduction to the MySQL Database Server, see Chapter 3, Tutorial.

- For information about configuring and administering MySQL Server, see Chapter 5, MySQL Server Administration.

- For information about setting up replication servers, see Chapter 6, Replication.

- For a list of currently known bugs and misfeatures, see Section A.1.8, “Known Issues in MySQL”.

- For a list of all the contributors to this project, see Appendix D, Credits.

- For a history of new features and bugfixes, see Appendix B, MySQL Change History.

- For tips on porting the MySQL Database Software to new architectures or operating systems, see MySQL Internals: Porting.

- For benchmarking information, see the `sql-bench` benchmarking directory in your MySQL distribution.

### Important

To report errors (often called “bugs”), please use the instructions at Section 1.7, “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 `<security@mysql.com>`.

## 1.1. About This Manual

This is the Reference Manual for all releases of the MySQL Database System from version 3.23 through release 4.1.24. It is also applicable for versions of the MySQL software previous to 4.1 (such as 3.23 or 4.0) because functional changes are indicated with reference to version numbers. For later MySQL releases, see the appropriately-numbered edition of this manual. For example, if you are using a version 5.0 release of the MySQL software, please refer to the MySQL 5.0 Reference Manual.

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 there, including HTML, PDF, and Windows CHM versions.

The Reference Manual source files are written in DocBook XML format. The HTML version and other formats are produced automatically, primarily using the DocBook XSL stylesheets. For information about DocBook, see http://docbook.org/

The DocBook XML sources of this manual are available from http://dev.mysql.com/tech-resources/sources.html. You can check out a copy of the documentation repository with this command:

    svn checkout http://svn.mysql.com/svnpublic/mysqldoc/

If you have questions about using MySQL, you can ask them using our mailing lists or forums. See Section 1.6.1, “MySQL Mailing Lists”, and Section 1.6.2, “MySQL Community Support at the MySQL Forums”. If you have suggestions concerning additions or corrections to the manual itself, please send them to the documentation team at `<docs@mysql.com>`.

This manual was originally written by David Axmark and Michael “Monty” Widenius. It is maintained by the MySQL Documentation Team, consisting of Paul DuBois, Stefan Hinz, Jon Stephens, Martin MC Brown, and Peter Lavin. For the many other contributors, see Appendix D, 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; program listings and source code; and environment variables. Example: “To reload the grant tables, use the `FLUSH PRIVILEGES` statement.”

- `Text in this style` indicates input that you type in examples.

- Text in this style indicates 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 prompt shown preceding the command indicates which command to use. 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 any lettercase. This manual uses uppercase.

In syntax descriptions, square brackets (“`[`” and “`]`”) 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, `SELECT ... INTO OUTFILE` is shorthand for the form of `SELECT` statement that has an `INTO OUTFILE` clause following other parts of the 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 the `CC` environment variable and run the configure command looks like this in Bourne shell syntax:

    shell> CC=gcc ./configure

If you are using csh or tcsh, you must issue commands somewhat differently:

    shell> setenv CC gcc
    shell> ./configure

## 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 United States and Germany, respectively.

## 1.4. Overview of the MySQL Database Management System

### 1.4.1. What is MySQL?

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 standalone 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.

### 1.4.2. History of MySQL

We started out with the intention of using the `mSQL` database system 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.

MySQL is named after co-founder Monty Widenius's daughter, My.

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.

### 1.4.3. The Main Features of MySQL

This section describes some of the important characteristics of the MySQL Database Software. See also Section 1.5, “MySQL Development Roadmap”, for more information about current and upcoming features. In most respects, it applies to all versions of MySQL. For information about features as they are introduced into MySQL on a series-specific basis, see the “In a Nutshell” section of the appropriate Manual:

- MySQL 4.0 and 4.1: MySQL 4.0 in a Nutshell, and MySQL 4.1 in a Nutshell.

- MySQL 5.0: MySQL 5.0 in a Nutshell.

- MySQL 5.1: MySQL 5.1 in a Nutshell.

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 On Which MySQL Is Known To Run”.

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

- The MySQL Server design is multi-layered with independent modules.

- 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 other storage engines. This is useful if you want to provide an SQL interface for 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.

Data Types:

- Many data 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 10, Data Types.

- Fixed-length and variable-length records.

Statements and Functions:

- Full operator and function support in the `SELECT` list and `WHERE` clause 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` statement can be used to retrieve information about databases, storage engines, tables, and indexes. MySQL 5.0 adds support for the `INFORMATION_SCHEMA` database, implemented according to standard SQL.

- The `EXPLAIN` statement 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 8.3, “Reserved Words”.

- You can refer to tables from different databases in the same statement.

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 (767 for `InnoDB`); before MySQL 4.1.2, the limit is 500 bytes. An index may use a prefix of a column for `CHAR`, `VARCHAR`, `BLOB`, or `TEXT` column types.

Connectivity:

- Clients can connect to MySQL Server using several protocols:

  - Clients can connect using TCP/IP sockets on any platform.

  - On Windows systems in the NT family (NT, 2000, XP, 2003, or Vista), clients can connect using named pipes if the server is started with the `--enable-named-pipe` option. In MySQL 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.

  - On Unix systems, clients can connect using Unix domain socket files.

- MySQL client programs can be written in many languages. A client library written in C is available for clients written in C or C++, or for any language that provides C bindings.

- APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl are available, allowing MySQL clients to be written in many languages. See Chapter 16, APIs and Libraries.

- 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 17, 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 17, Connectors.

- MySQL Connector/NET enables developers to easily create .NET applications that require secure, high-performance data connectivity with MySQL. It implements the required ADO.NET interfaces and integrates into ADO.NET aware tools. Developers can build applications using their choice of .NET languages. MySQL Connector/NET is a fully managed ADO.NET driver written in 100% pure C#. See Chapter 17, Connectors.

Localization:

- The server can provide error messages to clients in many languages. See Section 9.3, “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.

- Sorting and comparisons are done according to the chosen character set and collation (using `latin1` and 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.

- As of MySQL 4.1, the server time zone can be changed dynamically, and individual clients can specify their own time zone. Section 9.9, “MySQL Server Time Zone Support”.

MySQL Enterprise For assistance in getting optimal performance from your MySQL server subscribe to MySQL Enterprise. For more information see http://www.mysql.com/products/enterprise/.

Clients and Tools:

- MySQL AB provides several client and utility programs. These include both command-line programs such as mysqldump and mysqladmin, and graphical programs such as MySQL Administrator and MySQL Query Browser.

- 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 4, MySQL Programs.

- MySQL programs can be invoked with the `--help` or `-?` option to obtain online assistance.

## 1.5. MySQL Development Roadmap

This section describes the general MySQL development roadmap, including major features implemented in or planned for 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 MySQL 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 MySQL 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 currently is 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 in Section 2.11, “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:

Feature | MySQL Series
Unions | 4.0
Subqueries | 4.1
R-trees | 4.1 (for the `MyISAM` storage engine)
Stored procedures | 5.0
Views | 5.0
Cursors | 5.0
XA transactions | 5.0
Triggers | 5.0 and 5.1
Event scheduler | 5.1
Partitioning | 5.1
Pluggable storage engine API | 5.1
Plugin API | 5.1
Row-based replication | 5.1
Server log tables | 5.1
Foreign keys | 5.2 (implemented in 3.23 for `InnoDB`)

### 1.5.1. MySQL 4.0 in a Nutshell

MySQL 4.0 is available for download at http://dev.mysql.com/ and from our mirrors. MySQL 4.0 has been tested by a large number of users and is in production use at many large sites.

The following features were added in MySQL 4.0:

- Speed enhancements

  - MySQL 4.0 implemented a query cache that can give a major speed boost to applications with repetitive queries. See Section 7.5.3, “The MySQL Query Cache”.

  - MySQL 4.0 further increased the speed of MySQL Server in a number of areas, such as bulk `INSERT` statements, searching on packed indexes, full-text searching (using `FULLTEXT` indexes), and `COUNT(DISTINCT)`.

- `InnoDB` storage engine as standard

  - The `InnoDB` storage engine began to be offered as a standard feature of the MySQL server. This provided full support for ACID transactions, foreign keys with cascading `UPDATE` and `DELETE`, and row-level locking as standard features. See Section 13.2, “The `InnoDB` Storage Engine”.

- New functionality

  - The enhanced `FULLTEXT` search capabilities of MySQL Server 4.0 enabled `FULLTEXT` indexing of large text masses with both binary and natural-language searching logic. It became possible to customize minimal word length and define your own stop word lists in most human languages, enabling a broader class of applications to be built with MySQL Server. See Section 11.8, “Full-Text Search Functions”.

- Standards compliance, portability, and migration

  - MySQL Server added support for the `UNION` statement, a standard SQL feature.

  - Starting with version 4.0, MySQL runs natively on Novell NetWare 6.0 and higher. See Section 2.7, “Installing MySQL on NetWare”.

  - Features to simplify migration from other database systems to MySQL Server include `TRUNCATE TABLE` (as in Oracle) .

- Internationalization

  - German-speaking users should note that MySQL 4.0 added support for a new character set, `latin1_de`, which ensures that words with umlauts are sorted in the same order as in German telephone books.

- Usability enhancements

  - As of version 4.0, most mysqld parameters (startup options) can be set without taking down the server. This is a convenient feature for database administrators. See Section 12.5.3, “`SET` Syntax”.

  - Multiple-table `DELETE` and `UPDATE` statements were added.

  - On Windows, symbolic link handling at the database level was enabled by default. On Unix, the `MyISAM` storage engine added support for symbolic linking at the table level (and not just the database level as before).

  - The addition of the `SQL_CALC_FOUND_ROWS` and `FOUND_ROWS()` functions made it possible to find out the number of rows a `SELECT` query that includes a `LIMIT` clause would have returned without that clause.

- The Embedded MySQL Server

  The embedded server library added in this release can easily be used to create standalone and embedded applications. The embedded server provides an alternative to using MySQL in a client/server environment.

  The `libmysqld` embedded server library made MySQL Server suitable for a wider range of applications. Using this library, developers can embed MySQL Server into various applications and electronics devices, where the end user has no knowledge of there actually being an underlying database. Embedded MySQL Server is ideal for use in Internet appliances, public kiosks, turnkey hardware/software combination units, high performance Internet servers, self-contained databases distributed on CD-ROM, and so on.

  The embedded MySQL library uses the same interface as the normal client library. See Section 16.1, “libmysqld, the Embedded MySQL Server Library”. Embedded MySQL is available under the same dual-licensing model as the MySQL Server; see http://www.mysql.com/company/legal/licensing/ for more information.

  On Windows there are two different libraries:

  `libmysqld.lib` | Dynamic library for threaded applications.
  `mysqldemb.lib` | Static library for not threaded applications.

The news section of this manual includes a more in-depth list of MySQL 4.0 features. See Section B.2, “Changes in release 4.0.x”.

### 1.5.2. MySQL 4.1 in a Nutshell

MySQL Server 4.0 laid the foundation for new features implemented in MySQL 4.1, such as subqueries and Unicode support, which were desired by many of our customers.

MySQL Server 4.1 is currently in production status, and binaries are available for download at http://dev.mysql.com/downloads/mysql/4.1.html. All binary releases pass our extensive test suite without any errors on the platforms on which we test. See Section B.1, “Changes in release 4.1.x (Production)”.

For those wishing to use the most recent development source for MySQL 4.1, we also make our BitKeeper repositories publicly available. See Section 2.9.3, “Installing from the Development Source Tree”.

The following features are implemented in MySQL 4.1. Features that are available in MySQL 5.0 are described in Section 1.5.3, “What's New in MySQL 5.0”.

- Support for 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 12.2.8, “Subquery Syntax”.

- Speed enhancements:

  - Faster binary client/server protocol with support for prepared statements and parameter binding. See Section 16.2.4, “C API Prepared Statements”.

  - `BTREE` indexing is supported for `HEAP` tables, significantly improving response time for non-exact searches.

- Added functionality:

  - `CREATE TABLE `tbl_name2` LIKE `tbl_name1`` allows you to create, with a single statement, a new table with a structure exactly like that of an existing table.

  - The `MyISAM` storage engine added support for OpenGIS spatial types for storing geographical data. See Chapter 15, Spatial Extensions.

  - Support was added for replication over SSL connections.

  - Support for a number of additional storage engines was implemented in the MySQL 4.1 release series:

    - The `EXAMPLE` storage engine is a “stub” engine that serves as an example in the MySQL source code for writing new storage engines, and is primarily of interest to developers. See Section 13.6, “The `EXAMPLE` Storage Engine”.

    - `NDB Cluster` is the storage engine used by MySQL Cluster to implement tables that are partitioned over many computers. See Chapter 14, MySQL Cluster.

    - The `ARCHIVE` storage engine is used for storing large amounts of data without indexes in a very small footprint. See Section 13.7, “The `ARCHIVE` Storage Engine”.

    - The `CSV` storage engine stores data in text files using comma-separated values format. See Section 13.8, “The `CSV` Storage Engine”.

    - The `BLACKHOLE` storage engine accepts but does not store data, and always returns an empty result set. It is for use primarily in replication. See Section 13.9, “The `BLACKHOLE` Storage Engine”.

    ### Note

    These engine were implemented at different points in the development of MySQL 4.1. Please see the indicated sections for particulars in each case.

- Standards compliance, portability, and migration:

  - The enhanced client/server protocol available beginning with MySQL 4.1.1 provides the ability to pass multiple warnings to the client, rather than only a single result, making it much easier to track problems that occur in operations such as bulk data loading.

  - `SHOW WARNINGS` shows warnings for the last command. See Section 12.5.4.21, “`SHOW WARNINGS` Syntax”.

- Internationalization and Localization:

  - To support applications that require the use of local languages, the MySQL software added extensive Unicode support through the `utf8` and `ucs2` character sets.

  - Definition of character sets by column, table, and database. This allows for a high degree of flexibility in application design, particularly for multi-language Web sites. See Section 9.1, “Character Set Support”.

  - Per-connection time zones support, allowing individual clients to select their own time zones when necessary.

- Usability enhancements:

  - The addition of a server-based `HELP` command that can be used to get help information for SQL statements. This information is always applicable to the particular server version being used. Because this information is available by issuing an SQL statement, any client can access it. For example, the `help` command of the mysql command-line client has been modified to have this capability.

  - The improved client/server protocol allows multiple statements to be issued with a single call, and for returning multiple result sets. See Section 16.2.9, “C API Handling of Multiple Statement Execution”.

  - The syntax `INSERT ... ON DUPLICATE KEY UPDATE ...` was implemented. This allows you to update an existing row if the insert would have caused a duplicate value for a primary or unique index. See Section 12.2.4, “`INSERT` Syntax”.

  - The aggregate function `GROUP_CONCAT()`, added the capability to concatenate column values from grouped rows into a single result string. See Section 11.11, “Functions and Modifiers for Use with `GROUP BY` Clauses”.

The News section of this manual includes a more in-depth list of MySQL 4.1 features. See Section B.1, “Changes in release 4.1.x (Production)”.

### 1.5.3. 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.

- Cursors: Elementary support for server-side cursors.

- Data Dictionary (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.

- Instance Manager: Can be used to start and stop the MySQL Server, even from a remote host.

- 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.

- Storage Engines: Storage engines added in MySQL 5.0 include `ARCHIVE` and `FEDERATED`.

- Stored Routines: Support for named stored procedures and stored functions was implemented in MySQL 5.0.

- 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.

- Triggers: MySQL 5.0 added limited support for triggers.

- `VARCHAR` Data Type: The effective maximum length of a `VARCHAR` column was increased to 65,535 bytes, and stripping of trailing whitespace was eliminated. (The actual maximum length of a `VARCHAR` is determined by the maximum row size and the character set you use. The maximum effective column length is subject to a row size of 65,535 bytes.)

- Views: MySQL 5.0 added support for named, updatable views.

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 2.9.3, “Installing from the Development Source Tree”.

## 1.6. MySQL Information Sources

This section lists sources of additional information that you may find helpful, such as the MySQL mailing lists and user forums, and Internet Relay Chat.

### 1.6.1. 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.

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, “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 such as 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 graphical user interface tools such as `MySQL Administrator` and `MySQL Query Browser`.

- `cluster`

  This list is for discussion of MySQL Cluster.

- `dotnet`

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

- `plusplus`

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

- `perl`

  This list is for all topics concerning 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.

- ``<mysql-france-subscribe@yahoogroups.com>``

  A French mailing list.

- ``<list@tinc.net>``

  A Korean mailing list. To subscribe, email `subscribe mysql your@email.address` to this list.

- ``<mysql-de-request@lists.4t2.com>``

  A German mailing list. To subscribe, email `subscribe mysql-de your@email.address` to this list. You can find information about this mailing list at http://www.4t2.com/mysql/.

- ``<mysql-br-request@listas.linkway.com.br>``

  A Portuguese mailing list. To subscribe, email `subscribe mysql-br your@email.address` to this list.

- ``<mysql-alta@elistas.net>``

  A Spanish mailing list. To subscribe, email `subscribe mysql your@email.address` to this list.

#### 1.6.1.1. Guidelines for Using the Mailing Lists

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

When you answer a question sent to a mailing list, if you consider your answer to have broad interest, you may want to post it to the 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.

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.

### 1.6.2. MySQL Community Support at the MySQL Forums

The forums at http://forums.mysql.com are an important community resource. Many forums are available, grouped into these general categories:

- Migration

- MySQL Usage

- MySQL Connectors

- Programming Languages

- Tools

- 3rd-Party Applications

- Storage Engines

- MySQL Technology

- SQL Standards

- Business

### 1.6.3. MySQL Community Support on Internet Relay Chat (IRC)

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

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

- `#mysql` is primarily for 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/).

### 1.6.4. MySQL Enterprise

MySQL AB offers technical support in the form of MySQL Enterprise. For organizations that rely on the MySQL DBMS for business-critical production applications, MySQL Enterprise is a commercial subscription offering which includes:

- MySQL Enterprise Server

- MySQL Enterprise Monitor

- Monthly Rapid Updates and Quarterly Service Packs

- MySQL Knowledge Base

- 24x7 Technical and Consultative Support

MySQL Enterprise is available in multiple tiers, giving you the flexibility to choose the level of service that best matches your needs. For more information see MySQL Enterprise.

## 1.7. How to Report Bugs or Problems

Before posting a bug report about a problem, please try to verify that it is a bug and that it has not been reported already:

- Start by searching the MySQL online manual at http://dev.mysql.com/doc/. We try to keep the manual up to date by updating it frequently with solutions to newly found problems. The change history (http://dev.mysql.com/doc/mysql/en/news.html) can be particularly useful since it is quite possible that a newer version contains a solution to your problem.

- If you get a parse error for a SQL statement, 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 doesn't cover the syntax that you are using, MySQL Server doesn't support your statement. In this case, your options are to implement the syntax yourself or email `<licensing@mysql.com>` 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.

- For solutions to some common problems, see Section A.1, “Problems and Common Errors”.

- Search the bugs database at http://bugs.mysql.com/ to see whether the bug has been reported and fixed.

- Search the MySQL mailing list archives at http://lists.mysql.com/. See Section 1.6.1, “MySQL Mailing Lists”.

- You can also use http://www.mysql.com/search/ to search all the Web pages (including the manual) that are located at the MySQL AB Web site.

If you can't find an answer in the manual, the bugs database, or the mailing list archives, check with your local MySQL expert. If you still can't find an answer to your question, please use the following guidelines for reporting the bug.

The normal way to report bugs is to visit 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. If you have no Web access, you can generate a bug report by using the mysqlbug script described at the end of this section.

Bugs posted in the bugs database at http://bugs.mysql.com/ that are corrected for a given release are noted in the change history.

If you have found a sensitive security bug in MySQL, you can send email to `<security@mysql.com>`.

To discuss problems with other users, you can use one of the MySQL mailing lists. Section 1.6.1, “MySQL Mailing Lists”.

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. 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 your test case or by running the shell or Perl script that you include in the bug report. Any bug that we are able to repeat has a high chance of being fixed in the next MySQL release.

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 MySQL Internals: Porting.

Remember that it is possible for us to respond to a report 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 to follow is that 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 that you use, and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and version number). These are highly relevant pieces of information, and in 99 cases out of 100, the bug report is useless without them. 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. Errors often are 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 used. Note that every compiling problem should be regarded as a bug and reported accordingly.

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, it is better that the error message reported exactly matches the one that the program produces. (Even the lettercase should be observed.) It is best to copy and paste the entire error message into your report. You should never try to reproduce the message from memory.

If you have a problem with Connector/ODBC (MyODBC), please try to generate a trace file and send it with your report. See the MyODBC section of Chapter 17, Connectors.

If your report includes long query output lines from test cases that you run with the mysql command-line tool, you can make the output more readable by using the `--vertical` option or the `\G` statement terminator. The `EXPLAIN SELECT` example later in this section demonstrates the use of `\G`.

Please include the following information in your report:

- The version number of the MySQL distribution you are using (for example, MySQL 5.0.19). 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, include the name and version number of the compiler that you used. If you have a binary distribution, include the distribution name.

- 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 statement that crashed mysqld. You can usually get this information by running mysqld with query logging enabled, and then looking in the log after mysqld crashes. See MySQL Internals: Porting.

- If a database table is related to the problem, include the output from the `SHOW CREATE TABLE `db_name`.`tbl_name`` statement in the bug report. This is a very easy way to get the definition of any table in a database. The information helps us create a situation matching the one that you have experienced.

- The SQL mode in effect when the problem occurred can be significant, so please report the value of the `sql_mode` system variable. For stored procedure, stored function, and trigger objects, the relevant `sql_mode` value is the one in effect when the object was created. For a stored procedure or function, the `SHOW CREATE PROCEDURE` or `SHOW CREATE FUNCTION` statement shows the relevant SQL mode, or you can query `INFORMATION_SCHEMA` for the information:

      SELECT ROUTINE_SCHEMA, ROUTINE_NAME, SQL_MODE
      FROM INFORMATION_SCHEMA.ROUTINES;

  For triggers, you can use this statement:

      SELECT EVENT_OBJECT_SCHEMA, EVENT_OBJECT_TABLE, TRIGGER_NAME, SQL_MODE
      FROM INFORMATION_SCHEMA.TRIGGERS;

- For performance-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 table that is involved. The more information you provide about your situation, the more likely it is that someone can help you.

  The following is an example of a very good bug report. The statements are run using the mysql command-line tool. Note the use of the `\G` statement terminator for statements that would otherwise provide very long output lines that are difficult to read.

      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 upload it to be attached to the bug report.

  If you can't provide a script, you should at least include the output from mysqladmin variables extended-status processlist in your report 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 included in the bug report (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 believe that the MySQL server produces a strange result from a statement, include not only the result, but also your opinion of what the result should be, and an explanation describing the basis for your opinion.

- When you provide an example of the problem, it's better to use the table names, variable names, and so forth that exist in your actual situation than to come up with new names. The problem could be related to the name of a table or variable. 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. If you have data that you don't want to be visible to others in the bug report, 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, 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 key to resolving problems and are very relevant. It is never a bad idea to include them. If your problem involves a program written in a language such as Perl or PHP, please include the language processor's version number, as well as the version for any modules that the program uses. For example, if you have a Perl script that uses the `DBI` and `DBD::mysql` modules, include the version numbers for Perl, `DBI`, and `DBD::mysql`.

- 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 the exact purpose of the patch, 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 the problem is that your data appears corrupt or you get errors when you access a particular table, you should first check your tables and then try to repair them with `CHECK TABLE` and `REPAIR TABLE` or with myisamchk. See Chapter 5, MySQL Server 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. This variable affects how the server handles lettercase of database and table names. Its effect for a given value should be as described in Section 8.2.2, “Identifier Case Sensitivity”.

- 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 5.3.1, “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.1.1, “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 have no Web access and cannot report a bug by visiting http://bugs.mysql.com/, you can use the mysqlbug script to generate a bug report (or a report about any problem). mysqlbug 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. mysqlbug can be found in the `scripts` directory (source distribution) and in the `bin` directory under your MySQL installation directory (binary distribution).

## 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 of the 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” or “standard SQL” to mean the current version of the SQL Standard at any time.

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 is an example of this strategy. See Section 12.2.3, “`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-sized databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-sized 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 real-time support, although MySQL replication capabilities offer significant functionality.

In MySQL 4.1.2 in later, high-availability database clustering is supported by the `NDBCluster` storage engine. See Chapter 14, MySQL Cluster.

XML support is to be implemented in a future version of the database server.

### 1.8.1. What Standards MySQL Follows

Our aim is to support 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 capability enables each application to tailor the server's operating mode to its own requirements.

SQL modes control aspects of server operation such as what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in 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="`mode_value`"` option. Beginning with MySQL 4.1, you can also change the mode at runtime by setting the `sql_mode` system variable with a `SET [SESSION|GLOBAL] sql_mode='`mode_value`'` statement.

For more information on setting the SQL mode, see Section 5.1.6, “SQL Modes”.

### 1.8.3. Running MySQL in ANSI Mode

You can tell mysqld to run in ANSI mode with the `--ansi` startup option. Running the server in ANSI mode is the same as starting it with the following options:

    --transaction-isolation=SERIALIZABLE --sql-mode=ANSI

As of MySQL 4.1.1, you can achieve the same effect at runtime by executing these two statements:

    SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
    SET GLOBAL sql_mode = 'ANSI';

You can see that setting the `sql_mode` system variable to `'ANSI'` enables all SQL mode options that are relevant for ANSI mode as follows:

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

Note that running the server in ANSI mode with `--ansi` is not quite the same as setting the SQL mode to `'ANSI'`. The `--ansi` option affects the SQL mode and also sets the transaction isolation level. Setting the SQL mode to `'ANSI'` has no effect on the isolation level.

See Section 5.1.2, “Command Options”, and Section 1.8.2, “Selecting SQL Modes”.

### 1.8.4. MySQL Extensions to Standard SQL

MySQL Server supports 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 following form:

    /*! MySQL-specific code */

In this case, MySQL Server parses and executes the code within the comment as it would any other SQL statement, but other SQL servers will ignore the extensions. For example, MySQL Server recognizes the `STRAIGHT_JOIN` keyword in the following statement, but other servers will not:

    SELECT /*! STRAIGHT_JOIN */ col1 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 greater than or equal to the specified version number. The `TEMPORARY` keyword in the following comment is executed only by servers from MySQL 3.23.02 or higher:

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

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 maps tables within a database to filenames in the database directory. This has a few implications:

  - Database and table names are case sensitive in MySQL Server on operating systems that have case-sensitive filenames (such as most Unix systems). See Section 8.2.2, “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, it is possible to rename a `MyISAM` table by renaming the `.MYD`, `.MYI`, and `.frm` files to which the table corresponds. (Nevertheless, it is preferable to use `RENAME TABLE` or `ALTER TABLE ... RENAME` and let the server rename the files.)

  Database and table names cannot contain pathname separator characters (“`/`”, “`\`”).

- General language syntax

  - By default, strings can be enclosed by either “`"`” or “`'`”, not just by “`'`”. (If the `ANSI_QUOTES` SQL mode is enabled, strings can be enclosed only by “`'`” and the server interprets strings enclosed by “`"`” as identifiers.)

  - “`\`” is the 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

  - The `ANALYZE TABLE`, `CHECK TABLE`, `OPTIMIZE TABLE`, and `REPAIR TABLE` statements.

  - The `CREATE DATABASE`, `DROP DATABASE`, and `ALTER DATABASE` statements. See Section 12.1.3, “`CREATE DATABASE` Syntax”, Section 12.1.6, “`DROP DATABASE` Syntax”, and Section 12.1.1, “`ALTER DATABASE` Syntax”.

  - The `DO` statement.

  - `EXPLAIN SELECT` to obtain a description of how tables are processed by the query optimizer.

  - The `FLUSH` and `RESET` statements.

  - The `SET` statement. See Section 12.5.3, “`SET` Syntax”.

  - The `SHOW` statement. See Section 12.5.4, “`SHOW` Syntax”.

  - Use of `LOAD DATA INFILE`. In many cases, this syntax is compatible with Oracle's `LOAD DATA INFILE`. See Section 12.2.5, “`LOAD DATA INFILE` Syntax”.

  - Use of `RENAME TABLE`. See Section 12.1.9, “`RENAME TABLE` Syntax”.

  - Use of `REPLACE` instead of `DELETE` plus `INSERT`. See Section 12.2.6, “`REPLACE` Syntax”.

  - Use of `CHANGE `col_name``, `DROP `col_name``, or `DROP INDEX`, `IGNORE` or `RENAME` in `ALTER TABLE` statements. Use of multiple `ADD`, `ALTER`, `DROP`, or `CHANGE` clauses in an `ALTER TABLE` statement. See Section 12.1.2, “`ALTER TABLE` Syntax”.

  - Use of index names, indexes on a prefix of a column, and use of `INDEX` or `KEY` in `CREATE TABLE` statements. See Section 12.1.5, “`CREATE TABLE` Syntax”.

  - Use of `TEMPORARY` or `IF NOT EXISTS` with `CREATE TABLE`.

  - Use of `IF EXISTS` with `DROP TABLE` and `DROP DATABASE`.

  - The capability of dropping multiple tables with a single `DROP TABLE` statement.

  - The `ORDER BY` and `LIMIT` clauses of the `UPDATE` and `DELETE` statements.

  - `INSERT INTO `tbl_name` SET `col_name` = ...` syntax.

  - The `DELAYED` clause of the `INSERT` and `REPLACE` statements.

  - The `LOW_PRIORITY` clause of the `INSERT`, `REPLACE`, `DELETE`, and `UPDATE` statements.

  - Use of `INTO OUTFILE` or `INTO DUMPFILE` in `SELECT` statements. See Section 12.2.7, “`SELECT` Syntax”.

  - Options such as `STRAIGHT_JOIN` or `SQL_SMALL_RESULT` in `SELECT` statements.

  - You don't need to name all selected columns in the `GROUP BY` clause. This gives better performance for some very specific, but quite normal queries. See Section 11.11, “Functions and Modifiers for Use with `GROUP BY` Clauses”.

  - You can specify `ASC` and `DESC` with `GROUP BY`, not just with `ORDER BY`.

  - The ability to set variables in a statement with the `:=` assignment operator:

        mysql> SELECT @a:=SUM(total),@b:=COUNT(*),@a/@b AS avg
            -> FROM test_table;
        mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;

- Data types

  - The `MEDIUMINT`, `SET`, and `ENUM` data types, and the various `BLOB` and `TEXT` data types.

  - The `AUTO_INCREMENT`, `BINARY`, `NULL`, `UNSIGNED`, and `ZEROFILL` data type attributes.

- Functions and operators

  - To make it easier for users who migrate 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 `value_list`)` where `value_list` has more than one element.

  - String comparisons are case-insensitive by default, with sort ordering determined by the collation of the current character set, which is `latin1` (cp1252 West European) 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 expressions in the output column list (to the left of the `FROM`) in `SELECT` statements. For example:

        mysql> SELECT col1=1 AND col2=2 FROM my_table;

  - The `LAST_INSERT_ID()` function returns the most recent `AUTO_INCREMENT` value. See Section 11.10.3, “Information Functions”.

  - `LIKE` is allowed on numeric values.

  - 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 a variable 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 11.11, “Functions and Modifiers for Use with `GROUP BY` Clauses”.

For a prioritized list indicating when new extensions are added to MySQL Server, you should consult the online MySQL development roadmap at http://dev.mysql.com/doc/mysql/en/roadmap.html.

### 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:

- For `VARCHAR` columns, trailing spaces are removed when the value is stored. See Section A.1.8, “Known Issues in MySQL”.

- In some cases, `CHAR` columns are silently converted to `VARCHAR` columns when you define a table or alter its structure. See Section 12.1.5.1, “Silent Column Specification Changes”.

- There are several differences between the MySQL and standard SQL privilege systems. For example, in MySQL, privileges for a table are not automatically revoked when you delete a table. You must explicitly issue a `REVOKE` statement to revoke privileges for a table. For more information, see Section 12.5.1.3, “`REVOKE` Syntax”.

- The `CAST()` function does not support cast to `REAL` or `BIGINT`. See Section 11.9, “Cast Functions and Operators”.

#### 1.8.5.1. Subquery Support

MySQL 4.1 and up 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 12.2.8, “Subquery Syntax”.

For MySQL versions older than 4.1, most subqueries can be rewritten using joins or other methods. See Section 12.2.8.11, “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 `SELECT ... INTO TABLE` Sybase SQL extension. Instead, MySQL Server supports the `INSERT INTO ... SELECT` standard SQL syntax, which is basically the same thing. See Section 12.2.4.1, “`INSERT ... SELECT` Syntax”. For example:

    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`.

#### 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. MySQL Cluster is also a transaction-safe storage engine. See Chapter 13, Storage Engines. For information about `InnoDB` differences from standard SQL with regard to treatment of transaction errors, see Section 13.2.16, “`InnoDB` Error Handling”.

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.

Because MySQL Server supports 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 storage engines lies mostly in performance. Transactional tables have significantly higher memory and disk space requirements, and more CPU overhead. On the other hand, transactional storage engines 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 storage engines?

- 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.

- 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.

- To be safe with MySQL Server, regardless of whether you use transactional tables, you only need to have backups and have binary logging turned on. When that is true, 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 necessary, 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 write inserts that go into a local queue until the locks are released, without having the client wait for the insert to complete. See Section 7.3.3, “Concurrent Inserts”, and Section 12.2.4.2, “`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 employ 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 the conditions are satisfied.

  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 that 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 updating 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` 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 11.10.3, “Information Functions”, and Section 16.2.3.35, “`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. Otherwise, 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 this as though MySQL Server changed the preceding statement to:

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

#### 1.8.5.4. Stored Routines and Triggers

Stored procedures and functions are implemented beginning with MySQL 5.0.

Basic trigger functionality is implemented beginning with MySQL 5.0.2, with further development planned for MySQL 5.1.

#### 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 13.2.7.4, “`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 storage engine instead, such as `MyISAM`. (For example, the `MyISAM` storage engine offers very fast performance for applications that perform only `INSERT` and `SELECT` operations. In this case, the table has no holes in the middle and the inserts can be performed concurrently with retrievals. See Section 7.3.3, “Concurrent Inserts”.)

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, you can achieve a similar effect as of MySQL Server 4.0 by using multiple-table `DELETE` statements to delete rows from many tables with a single statement. See Section 12.2.1, “`DELETE` Syntax”.

- A workaround for the lack of `ON DELETE` is to add the appropriate `DELETE` statements 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 sometimes 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 13.2.7.4, “`FOREIGN KEY` Constraints”. As of MySQL 4.1.1, mysqldump generates dump files that take advantage of this capability automatically when they are 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 INNER JOIN t2 ON t1.id = t2.id;

See Section 12.2.7.1, “`JOIN` Syntax”, and 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 beginning with MySQL Server 5.0.1.

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 5.5, “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

Standard SQL uses the C syntax `/* this is a comment */` for comments, and MySQL Server supports this syntax as well. MySQL also support extensions to this syntax that allow MySQL-specific SQL to be embedded in the comment, as described in Section 8.5, “Comment Syntax”.

Standard SQL uses “`--`” as a start-comment sequence. MySQL Server uses “`#`” as the start comment character. MySQL Server 3.23.3 and up also supports a variant of the “`--`” comment style. That is, the “`--`” start-comment sequence must be followed by a space (or by a control character such as a newline). The space is required to prevent problems with automatically generated SQL queries that use constructs such as the following, where we automatically insert the value of the payment for `payment`:

    UPDATE account SET credit=credit-payment

Consider about what happens if `payment` has a negative value such as `-1`:

    UPDATE account SET credit=credit--1

`credit--1` is a legal expression in SQL, but “`--`” 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 requires a space following the “`--`” in order for it to be recognized as a start-comment sequence in MySQL Server 3.23.3 and newer. Therefore, `credit--1` is safe to use.

Another safe feature is that the mysql command-line client ignores 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 script in a text file that contains “`--`” comments, you should use the replace utility as follows to convert the comments to use “`#`” characters before executing the script:

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

That is safer than executing the script in the usual way:

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

You can also edit the script 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

See Section 4.8.2, “replace — A String-Replacement Utility”.

### 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.

The following sections describe how MySQL Server handles different types of constraints.

#### 1.8.6.1. `PRIMARY KEY` and `UNIQUE` Index Constraints

Normally, errors occurs for data-change statements (such as `INSERT` or `UPDATE`) that would violate primary-key, unique-key, or foreign-key constraints. 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.

MySQL supports an `IGNORE` keyword for `INSERT`, `UPDATE`, and so forth. If you use it, MySQL ignores primary-key or unique-key violations and continues processing with the next row. See the section for the statement that you are using (Section 12.2.4, “`INSERT` Syntax”, Section 12.2.10, “`UPDATE` Syntax”, and so forth).

You can get information about the number of rows actually inserted or updated with the `mysql_info()` C API function. In MySQL 4.1 and up, you also can use the `SHOW WARNINGS` statement. See Section 16.2.3.33, “`mysql_info()`”, and Section 12.5.4.21, “`SHOW WARNINGS` Syntax”.

Currently, only `InnoDB` tables support foreign keys. See Section 13.2.7.4, “`FOREIGN KEY` Constraints”. We plan to add foreign key support by other storage engines in a future MySQL release. See Section 1.5, “MySQL Development Roadmap”.

#### 1.8.6.2. Constraints on Invalid Data

Through version 4.1, MySQL is forgiving of illegal or improper data values and coerces them to legal values for data entry. When 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. The following rules describe in more detail how this works:

- If you try to store an out of range value into a numeric column, MySQL Server instead stores zero, the smallest possible value, or the largest possible value, whichever is closest to the invalid value.

- 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.

- Invalid values for `ENUM` and `SET` columns are handled as described in Section 1.8.6.3, “`ENUM` and `SET` Constraints”.

- 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 “zero” 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 10.1.4, “Data Type Default Values”.

- 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 using 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.

#### 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. See Section 10.4.4, “The `ENUM` Type”, and Section 10.4.5, “The `SET` Type”. However, in MySQL 4.1 and earlier, `ENUM` and `SET` columns do not provide true constraints on entry of invalid data:

- `ENUM` columns always have a default value. If you specify no default value, then it is `NULL` for columns that can have `NULL`, otherwise it is the first enumeration value in the column definition.

- 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.

- 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'`.

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