SET variable_assignment [, variable_assignment] ...

variable_assignment:
      user_var_name = expr
    | param_name = expr
    | local_var_name = expr
    | [GLOBAL | SESSION | PERSIST | PERSIST_ONLY]
        system_var_name = expr
    | [@@global. | @@session. | @@persist. | @@persist_only. | @@]
        system_var_name = expr

SET syntax for variable assignment enables you to assign values to different types of variables that affect the operation of the server or clients:

A SET statement that assigns variable values is not written to the binary log, so in replication scenarios it affects only the host on which you execute it. To affect all replication hosts, execute the statement on each one.

The following examples illustrate SET syntax for setting variables. They use the = assignment operator, but the := assignment operator is also permitted for this purpose.

A user 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).

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

CREATE PROCEDURE p()
BEGIN
  DECLARE counter INT DEFAULT 0;
  WHILE counter < 10 DO
    -- ... do work ...
    SET counter = counter + 1;
  END WHILE;
END;

Many system variables are dynamic and can be changed at runtime by using the SET statement. For a list, see Section 5.1.6.2, “Dynamic System Variables”. To change a system variable with SET, refer to it by name, optionally preceded by a modifier:

The preceding modifiers apply only to system variables. An error occurs for attempts to apply them to user-defined variables, stored procedure or function parameters, or stored program local variables.

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, the most recent GLOBAL, PERSIST, PERSIST_ONLY, or SESSION modifier in the statement is used for following assignments that have no modifier 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;

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.

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 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 variable. However, the change affects the corresponding session variable only for clients that connect after the change. The global variable change does not affect the session variable for any current client sessions (not even the session within which the SET GLOBAL statement occurred).

To make a global system variable setting permanent so that it applies across server restarts, modify it with SET PERSIST or PERSIST_ONLY to record it in the mysqld-auto.cnf file. It is also possible to use SET GLOBAL and manually modify a my.cnf file, but that is more cumbersome, and an error in a manually entered setting might not be discovered until much later. SET PERSIST or PERSIST_ONLY is more convenient and avoids the possibility of malformed settings.

The Performance Schema persisted_variables table provides an SQL interface to the mysqld-auto.cnf file, enabling its contents to be inspected at runtime using SELECT statements. See Section 25.11.13.1, “Performance Schema persisted_variables Table”.

The Performance Schema variables_info table contains information showing when and by which user each system variable was most recently set. See Section 25.11.13.2, “Performance Schema variables_info Table”.

To set a GLOBAL value to the compiled-in MySQL default value or a SESSION 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;

Not all system variables can be set to DEFAULT. In such cases, assigning DEFAULT results in an error.

With SET PERSIST (or @@persist.), setting a global variable to 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.

An error occurs for attempts to assign DEFAULT to user-defined variables, stored procedure or function parameters, or stored program local variables.

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

SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;

For a reference to a system variable in an expression as @@var_name (rather than with @@global. or @@session.), MySQL returns the session value if it exists and the global value otherwise. This differs from SET @@var_name = expr, which always refers to the session value.