1.9.4. 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).

  • You can use standard system commands to back up, rename, move, delete, and copy tables that are managed by the MyISAM storage engine. 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.

  • 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.

  • The SHOW statement. As of MySQL 5.0, the information produced by many of the MySQL-specific SHOW statements can be obtained in more standard fashion by using SELECT to query INFORMATION_SCHEMA.

  • Use of LOAD DATA INFILE. In many cases, this syntax is compatible with Oracle's LOAD DATA INFILE.

  • Use of RENAME TABLE.

  • Use of REPLACE instead of DELETE plus INSERT.

  • 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.

  • Use of index names, indexes on a prefix of a column, and use of INDEX or KEY in CREATE TABLE statements.

  • 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.

  • 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.

  • 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.

  • 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 12.10, “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.