User-defined variables are created locally within a session and exist only within the context of that session; see Section 11.4, “User-Defined Variables”.

A user-defined variable is written as @var_name and is assigned an expression value as follows:

SET @var_name = expr;

Examples:

SET @name = 43;
SET @total_tax = (SELECT SUM(tax) FROM taxable_transactions);

As demonstrated by those statements, expr can range from simple (a literal value) to more complex (the value returned by a scalar subquery).

The Performance Schema user_variables_by_thread table contains information about user-defined variables. See Section 29.12.10, “Performance Schema User-Defined Variable Tables”.

SET applies to parameters and local variables in the context of the stored object within which they are defined. The following procedure uses the increment procedure parameter and counter local variable:

CREATE PROCEDURE p(increment INT)
BEGIN
  DECLARE counter INT DEFAULT 0;
  WHILE counter < 10 DO
    -- ... do work ...
    SET counter = counter + increment;
  END WHILE;
END;

The MySQL server maintains system variables that configure its operation. A system variable can have a global value that affects server operation as a whole, a session value that affects the current session, or both. Many system variables are dynamic and can be changed at runtime using the SET statement to affect operation of the current server instance. SET can also be used to persist certain system variables to the mysqld-auto.cnf file in the data directory, to affect server operation for subsequent startups.

If a SET statement is issued for a sensitive system variable, the query is rewritten to replace the value with “<redacted>” before it is logged to the general log and audit log. This takes place even if secure storage through a keyring component is not available on the server instance.

If you change a session system variable, the value remains in effect within your session until you change the variable to a different value or the session ends. The change has no effect on other sessions.

If you change a global system variable, the value is remembered and used to initialize the session value for new sessions until you change the variable to a different value or the server exits. The change is visible to any client that accesses the global value. However, the change affects the corresponding session value only for clients that connect after the change. The global variable change does not affect the session value for any current client sessions (not even the session within which the global value change occurs).

To make a global system variable setting permanent so that it applies across server restarts, you can persist it to the mysqld-auto.cnf file in the data directory. It is also possible to make persistent configuration changes by manually modifying a my.cnf option file, but that is more cumbersome, and an error in a manually entered setting might not be discovered until much later. SET statements that persist system variables are more convenient and avoid the possibility of malformed settings because settings with syntax errors do not succeed and do not change server configuration. For more information about persisting system variables and the mysqld-auto.cnf file, see Section 7.1.9.3, “Persisted System Variables”.

The following discussion describes the syntax options for setting and persisting system variables:

To set a global system variable value to the compiled-in MySQL default value or a session system variable to the current corresponding global value, set the variable to the value DEFAULT. For example, the following two statements are identical in setting the session value of max_join_size to the current global value:

SET @@SESSION.max_join_size = DEFAULT;
SET @@SESSION.max_join_size = @@GLOBAL.max_join_size;

Using SET to persist a global system variable to a value of DEFAULT or to its literal default value assigns the variable its default value and adds a setting for the variable to mysqld-auto.cnf. To remove the variable from the file, use RESET PERSIST.

Some system variables cannot be persisted or are persist-restricted. See Section 7.1.9.4, “Nonpersistible and Persist-Restricted System Variables”.

A system variable implemented by a plugin can be persisted if the plugin is installed when the SET statement is executed. Assignment of the persisted plugin variable takes effect for subsequent server restarts if the plugin is still installed. If the plugin is no longer installed, the plugin variable no longer exists when the server reads the mysqld-auto.cnf file. In this case, the server writes a warning to the error log and continues:

currently unknown variable 'var_name'
was read from the persisted config file

To display system variable names and values:

If any variable assignment in a SET statement fails, the entire statement fails and no variables are changed, nor is the mysqld-auto.cnf file changed.

SET produces an error under the circumstances described here. Most of the examples show SET statements that use keyword syntax (for example, GLOBAL or SESSION), but the principles are also true for statements that use the corresponding modifiers (for example, @@GLOBAL. or @@SESSION.).

A SET statement can contain multiple variable assignments, separated by commas. This statement assigns values to a user-defined variable and a system variable:

SET @x = 1, SESSION sql_mode = '';

If you set multiple system variables in a single statement, the most recent GLOBAL, PERSIST, PERSIST_ONLY, or SESSION keyword in the statement is used for following assignments that have no keyword specified.

Examples of multiple-variable assignment:

SET GLOBAL sort_buffer_size = 1000000, SESSION sort_buffer_size = 1000000;
SET @@GLOBAL.sort_buffer_size = 1000000, @@LOCAL.sort_buffer_size = 1000000;
SET GLOBAL max_connections = 1000, sort_buffer_size = 1000000;

The @@GLOBAL., @@PERSIST., @@PERSIST_ONLY., @@SESSION., and @@ modifiers apply only to the immediately following system variable, not any remaining system variables. This statement sets the sort_buffer_size global value to 50000 and the session value to 1000000:

SET @@GLOBAL.sort_buffer_size = 50000, sort_buffer_size = 1000000;

To refer to the value of a system variable in expressions, use one of the @@-modifiers (except @@PERSIST. and @@PERSIST_ONLY., which are not permitted in expressions). For example, you can retrieve system variable values in a SELECT statement like this:

SELECT @@GLOBAL.sql_mode, @@SESSION.sql_mode, @@sql_mode;