Syntax
------

SET variable_assignment [, variable_assignment] ...

variable_assignment:
   user_var_name = expr
  | [GLOBAL | SESSION] system_var_name = expr
  | [@@global. | @@session. | @@]system_var_name = expr

One can also set a user variable in any expression with this syntax:

user_var_name:= expr

Description
-----------

The SET statement assigns values to different types of variables that affect
the operation of the server or your client. Older versions of MySQL employed
SET OPTION, but this syntax was deprecated in favor of SET without OPTION, and
was removed in MariaDB 10.0.

Changing a system variable by using the SET statement does not make the change
permanently. To do so, the change must be made in a configuration file.

For setting variables on a per-query basis, see SET STATEMENT.

See SHOW VARIABLES for documentation on viewing server system variables.

See Server System Variables for a list of all the system variables.

GLOBAL / SESSION
----------------

When setting a system variable, the scope can be specified as either GLOBAL or
SESSION.

A global variable change affects all new sessions. It does not affect any
currently open sessions, including the one that made the change.

A session variable change affects the current session only.

If the variable has a session value, not specifying either GLOBAL or SESSION
will be the same as specifying SESSION. If the variable only has a global
value, not specifying GLOBAL or SESSION will apply to the change to the global
value.

DEFAULT
-------

Setting a global variable to DEFAULT will restore it to the server default,
and setting a session variable to DEFAULT will restore it to the current
global value.

Examples
--------

* innodb_sync_spin_loops is a global variable.
* skip_parallel_replication is a session variable.
* max_error_count is both global and session.

SELECT VARIABLE_NAME, SESSION_VALUE, GLOBAL_VALUE FROM
 INFORMATION_SCHEMA.SYSTEM_VARIABLES WHERE 
 VARIABLE_NAME IN ('max_error_count', 'skip_parallel_replication',
'innodb_sync_spin_loops');
+---------------------------+---------------+--------------+
| VARIABLE_NAME             | SESSION_VALUE | GLOBAL_VALUE |
+---------------------------+---------------+--------------+
| MAX_ERROR_COUNT           | 64            | 64           |
| SKIP_PARALLEL_REPLICATION | OFF           | NULL         |
| INNODB_SYNC_SPIN_LOOPS    | NULL          | 30           |
+---------------------------+---------------+--------------+

Setting the session values:

SET max_error_count=128;Query OK, 0 rows affected (0.000 sec)

SET skip_parallel_replication=ON;Query OK, 0 rows affected (0.000 sec)

SET innodb_sync_spin_loops=60;
ERROR 1229 (HY000): Variable 'innodb_sync_spin_loops' is a GLOBAL variable 
 and should be set with SET GLOBAL

SELECT VARIABLE_NAME, SESSION_VALUE, GLOBAL_VALUE FROM
 INFORMATION_SCHEMA.SYSTEM_VARIABLES WHERE 
 VARIABLE_NAME IN ('max_error_count', 'skip_parallel_replication',
'innodb_sync_spin_loops');
+---------------------------+---------------+--------------+
| VARIABLE_NAME             | SESSION_VALUE | GLOBAL_VALUE |
+---------------------------+---------------+--------------+
| MAX_ERROR_COUNT           | 128           | 64           |
| SKIP_PARALLEL_REPLICATION | ON            | NULL         |
| INNODB_SYNC_SPIN_LOOPS    | NULL          | 30           |
+---------------------------+---------------+--------------+

Setting the global values:

SET GLOBAL max_error_count=256;

SET GLOBAL skip_parallel_replication=ON;
ERROR 1228 (HY000): Variable 'skip_parallel_replication' is a SESSION variable 
 and can't be used with SET GLOBAL

SET GLOBAL innodb_sync_spin_loops=120;

SELECT VARIABLE_NAME, SESSION_VALUE, GLOBAL_VALUE FROM
 INFORMATION_SCHEMA.SYSTEM_VARIABLES WHERE 
 VARIABLE_NAME IN ('max_error_count', 'skip_parallel_replication',
'innodb_sync_spin_loops');
+---------------------------+---------------+--------------+
| VARIABLE_NAME             | SESSION_VALUE | GLOBAL_VALUE |
+---------------------------+---------------+--------------+
| MAX_ERROR_COUNT           | 128           | 256          |
| SKIP_PARALLEL_REPLICATION | ON            | NULL         |
| INNODB_SYNC_SPIN_LOOPS    | NULL          | 120          |
+---------------------------+---------------+--------------+

SHOW VARIABLES will by default return the session value unless the variable is
global only.

SHOW VARIABLES LIKE 'max_error_count';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_error_count | 128   |
+-----------------+-------+

SHOW VARIABLES LIKE 'skip_parallel_replication';
+---------------------------+-------+
| Variable_name             | Value |
+---------------------------+-------+
| skip_parallel_replication | ON    |
+---------------------------+-------+

SHOW VARIABLES LIKE 'innodb_sync_spin_loops';
+------------------------+-------+
| Variable_name          | Value |
+------------------------+-------+
| innodb_sync_spin_loops | 120   |
+------------------------+-------+

Using the inplace syntax:

SELECT (@a:=1);
+---------+
| (@a:=1) |
+---------+
|       1 |
+---------+

SELECT @a;
+------+
| @a   |
+------+
|    1 |
+------+

URL: https://mariadb.com/kb/en/set/