Using System Variables With a  LIKE  clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a  LIKE  clause as shown: To get a list of variables whose name match a pattern, use the  %  wildcard character in a  LIKE  clause: Wildcard characters can be used in any position within the pattern to be matched. Strictly speaking, because  _  is a wildcard that matches any single character, you should escape it as  \_  to match it literally. In practice, this is rarely necessary. For  SHOW VARIABLES , if you specify neither  GLOBAL  nor  SESSION , MySQL returns  SESSION  values. The reason for requiring the  GLOBAL  keyword when setting  GLOBAL -only variables but not when retrieving them is to prevent problems in the future: A system variable can have a global value that affects server operation as a whole, a session value that affects only the current session, or both: This section describes the privileges required for operations that assign values to system variables at runtime. This includes operations that affect runtime values, and operations that persist values. To set a global system variable, use a  SET  statement with the appropriate keyword. These privileges apply: •To set a global system variable runtime value, use the  SET GLOBAL  statement, which requires the SYSTEM_VARIABLES_ADMIN  privilege (or the deprecated  SUPER  privilege). •To persist a global system variable to the  mysqld-auto.cnf  file (and set the runtime value), use the SET PERSIST  statement, which requires the  SYSTEM_VARIABLES_ADMIN  or  SUPER  privilege. •To persist a global system variable to the  mysqld-auto.cnf  file (without setting the runtime value), use the  SET PERSIST_ONLY  statement, which requires the  SYSTEM_VARIABLES_ADMIN  and PERSIST_RO_VARIABLES_ADMIN  privileges.  SET PERSIST_ONLY  can be used for both dynamic and read-only system variables, but is particularly useful for persisting read-only variables, for which  SET PERSIST  cannot be used. 919 •For dynamic system variables, the  SET  statement can be used to change their global or session runtime value (or both), to affect operation of the current server instance. (For information about dynamic variables, see Section 5.1.9.2, “Dynamic System Variables” .) •For certain global system variables,  SET  can be used to persist their value to the  mysqld-auto.cnf  file in the data directory, to affect server operation for subsequent startups. (For information about persisting system variables and the  mysqld-auto.cnf  file, see Section 5.1.9.3, “Persisted System Variables” .) •For persisted global system variables,  RESET PERSIST  can be used to remove their value from mysqld-auto.cnf , to affect server operation for subsequent startups. •Were a  SESSION  variable to be removed that has the same name as a  GLOBAL  variable, a client with privileges sufficient to modify global variables might accidentally change the  GLOBAL  variable rather than just the  SESSION  variable for its own session. •Were a  SESSION  variable to be added with the same name as a  GLOBAL  variable, a client that intends to change the  GLOBAL  variable might find only its own  SESSION  variable changed. SHOW VARIABLES LIKE ’max_join_size’; SHOW SESSION VARIABLES LIKE ’max_join_size’; SHOW VARIABLES LIKE ’%size%’; SHOW GLOBAL VARIABLES LIKE ’%size%’; 5.1.9.1 System Variable Privileges