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13.1.13 CREATE EVENT Syntax

Inhoudsopgave
  1. CREATE
  2.     [DEFINER = user]
  3.     EVENT
  4.     [IF NOT EXISTS]
  5.     event_name
  6.     ON SCHEDULE schedule
  7.     [ON COMPLETION [NOT] PRESERVE]
  8.     [ENABLE | DISABLE | DISABLE ON SLAVE]
  9.     [COMMENT 'string']
  10.     DO event_body;
  11.  
  12. schedule:
  13.     AT timestamp [+ INTERVAL interval] ...
  14.   | EVERY interval
  15.     [STARTS timestamp [+ INTERVAL interval] ...]
  16.     [ENDS timestamp [+ INTERVAL interval] ...]
  17.  
  18. interval:
  19.     quantity {YEAR | QUARTER | MONTH | DAY | HOUR | MINUTE |
  20.               WEEK | SECOND | YEAR_MONTH | DAY_HOUR | DAY_MINUTE |
  21.               DAY_SECOND | HOUR_MINUTE | HOUR_SECOND | MINUTE_SECOND}

This statement creates and schedules a new event. The event will not run unless the Event Scheduler is enabled. For information about checking Event Scheduler status and enabling it if necessary, see Section 24.4.2, “Event Scheduler Configuration”.

CREATE EVENT requires the EVENT privilege for the schema in which the event is to be created. If the DEFINER clause is present, the privileges required depend on the user value, as discussed in Section 24.6, “Stored Object Access Control”.

The minimum requirements for a valid CREATE EVENT statement are as follows:

  • The keywords CREATE EVENT plus an event name, which uniquely identifies the event in a database schema.

  • An ON SCHEDULE clause, which determines when and how often the event executes.

  • A DO clause, which contains the SQL statement to be executed by an event.

This is an example of a minimal CREATE EVENT statement:

  1. CREATE EVENT myevent
  2.     ON SCHEDULE AT CURRENT_TIMESTAMP + INTERVAL 1 HOUR
  3.     DO
  4.       UPDATE myschema.mytable SET mycol = mycol + 1;

The previous statement creates an event named myevent. This event executes once—one hour following its creation—by running an SQL statement that increments the value of the myschema.mytable table's mycol column by 1.

The event_name must be a valid MySQL identifier with a maximum length of 64 characters. Event names are not case-sensitive, so you cannot have two events named myevent and MyEvent in the same schema. In general, the rules governing event names are the same as those for names of stored routines. See Section 9.2, “Schema Object Names”.

An event is associated with a schema. If no schema is indicated as part of event_name, the default (current) schema is assumed. To create an event in a specific schema, qualify the event name with a schema using schema_name.event_name syntax.

The DEFINER clause specifies the MySQL account to be used when checking access privileges at event execution time. If the DEFINER clause is present, the user value should be a MySQL account specified as 'user_name'@'host_name', CURRENT_USER, or CURRENT_USER(). The permitted user values depend on the privileges you hold, as discussed in Section 24.6, “Stored Object Access Control”. Also see that section for additional information about event security.

If the DEFINER clause is omitted, the default definer is the user who executes the CREATE EVENT statement. This is the same as specifying DEFINER = CURRENT_USER explicitly.

Within an event body, the CURRENT_USER function returns the account used to check privileges at event execution time, which is the DEFINER user. For information about user auditing within events, see Section 6.2.22, “SQL-Based Account Activity Auditing”.

IF NOT EXISTS has the same meaning for CREATE EVENT as for CREATE TABLE: If an event named event_name already exists in the same schema, no action is taken, and no error results. (However, a warning is generated in such cases.)

The ON SCHEDULE clause determines when, how often, and for how long the event_body defined for the event repeats. This clause takes one of two forms:

  • AT timestamp is used for a one-time event. It specifies that the event executes one time only at the date and time given by timestamp, which must include both the date and time, or must be an expression that resolves to a datetime value. You may use a value of either the DATETIME or TIMESTAMP type for this purpose. If the date is in the past, a warning occurs, as shown here:

    1. mysql> SELECT NOW();
    2. +---------------------+
    3. | NOW()               |
    4. +---------------------+
    5. | 2006-02-10 23:59:01 |
    6. +---------------------+
    7. 1 row in set (0.04 sec)
    8.  
    9. mysql> CREATE EVENT e_totals
    10.     ->     ON SCHEDULE AT '2006-02-10 23:59:00'
    11.     ->     DO INSERT INTO test.totals VALUES (NOW());
    12. Query OK, 0 rows affected, 1 warning (0.00 sec)
    13.  
    14. mysql> SHOW WARNINGS\G
    15. *************************** 1. row ***************************
    16.   Level: Note
    17.    Code: 1588
    18. Message: Event execution time is in the past and ON COMPLETION NOT
    19.          PRESERVE is set. The event was dropped immediately after
    20.          creation.

    CREATE EVENT statements which are themselves invalid—for whatever reason—fail with an error.

    You may use CURRENT_TIMESTAMP to specify the current date and time. In such a case, the event acts as soon as it is created.

    To create an event which occurs at some point in the future relative to the current date and time—such as that expressed by the phrase “three weeks from now”—you can use the optional clause + INTERVAL interval. The interval portion consists of two parts, a quantity and a unit of time, and follows the syntax rules described in Temporal Intervals, except that you cannot use any units keywords that involving microseconds when defining an event. With some interval types, complex time units may be used. For example, “two minutes and ten seconds” can be expressed as + INTERVAL '2:10' MINUTE_SECOND.

    You can also combine intervals. For example, AT CURRENT_TIMESTAMP + INTERVAL 3 WEEK + INTERVAL 2 DAY is equivalent to “three weeks and two days from now”. Each portion of such a clause must begin with + INTERVAL.

  • To repeat actions at a regular interval, use an EVERY clause. The EVERY keyword is followed by an interval as described in the previous discussion of the AT keyword. (+ INTERVAL is not used with EVERY.) For example, EVERY 6 WEEK means “every six weeks”.

    Although + INTERVAL clauses are not permitted in an EVERY clause, you can use the same complex time units permitted in a + INTERVAL.

    An EVERY clause may contain an optional STARTS clause. STARTS is followed by a timestamp value that indicates when the action should begin repeating, and may also use + INTERVAL interval to specify an amount of time “from now”. For example, EVERY 3 MONTH STARTS CURRENT_TIMESTAMP + INTERVAL 1 WEEK means “every three months, beginning one week from now”. Similarly, you can express “every two weeks, beginning six hours and fifteen minutes from now” as EVERY 2 WEEK STARTS CURRENT_TIMESTAMP + INTERVAL '6:15' HOUR_MINUTE. Not specifying STARTS is the same as using STARTS CURRENT_TIMESTAMP—that is, the action specified for the event begins repeating immediately upon creation of the event.

    An EVERY clause may contain an optional ENDS clause. The ENDS keyword is followed by a timestamp value that tells MySQL when the event should stop repeating. You may also use + INTERVAL interval with ENDS; for instance, EVERY 12 HOUR STARTS CURRENT_TIMESTAMP + INTERVAL 30 MINUTE ENDS CURRENT_TIMESTAMP + INTERVAL 4 WEEK is equivalent to “every twelve hours, beginning thirty minutes from now, and ending four weeks from now”. Not using ENDS means that the event continues executing indefinitely.

    ENDS supports the same syntax for complex time units as STARTS does.

    You may use STARTS, ENDS, both, or neither in an EVERY clause.

    If a repeating event does not terminate within its scheduling interval, the result may be multiple instances of the event executing simultaneously. If this is undesirable, you should institute a mechanism to prevent simultaneous instances. For example, you could use the GET_LOCK() function, or row or table locking.

The ON SCHEDULE clause may use expressions involving built-in MySQL functions and user variables to obtain any of the timestamp or interval values which it contains. You may not use stored functions or user-defined functions in such expressions, nor may you use any table references; however, you may use SELECT FROM DUAL. This is true for both CREATE EVENT and ALTER EVENT statements. References to stored functions, user-defined functions, and tables in such cases are specifically not permitted, and fail with an error (see Bug #22830).

Times in the ON SCHEDULE clause are interpreted using the current session time_zone value. This becomes the event time zone; that is, the time zone that is used for event scheduling and is in effect within the event as it executes. These times are converted to UTC and stored along with the event time zone in the mysql.event table. This enables event execution to proceed as defined regardless of any subsequent changes to the server time zone or daylight saving time effects. For additional information about representation of event times, see Section 24.4.4, “Event Metadata”. See also Section 13.7.6.18, “SHOW EVENTS Syntax”, and Section 25.10, “The INFORMATION_SCHEMA EVENTS Table”.

Normally, once an event has expired, it is immediately dropped. You can override this behavior by specifying ON COMPLETION PRESERVE. Using ON COMPLETION NOT PRESERVE merely makes the default nonpersistent behavior explicit.

You can create an event but prevent it from being active using the DISABLE keyword. Alternatively, you can use ENABLE to make explicit the default status, which is active. This is most useful in conjunction with ALTER EVENT (see Section 13.1.3, “ALTER EVENT Syntax”).

A third value may also appear in place of ENABLE or DISABLE; DISABLE ON SLAVE is set for the status of an event on a replication slave to indicate that the event was created on the master and replicated to the slave, but is not executed on the slave. See Section 17.4.1.16, “Replication of Invoked Features”.

You may supply a comment for an event using a COMMENT clause. comment may be any string of up to 64 characters that you wish to use for describing the event. The comment text, being a string literal, must be surrounded by quotation marks.

The DO clause specifies an action carried by the event, and consists of an SQL statement. Nearly any valid MySQL statement that can be used in a stored routine can also be used as the action statement for a scheduled event. (See Section C.1, “Restrictions on Stored Programs”.) For example, the following event e_hourly deletes all rows from the sessions table once per hour, where this table is part of the site_activity schema:

  1. CREATE EVENT e_hourly
  2.     ON SCHEDULE
  3.       EVERY 1 HOUR
  4.     COMMENT 'Clears out sessions table each hour.'
  5.     DO
  6.       DELETE FROM site_activity.sessions;

MySQL stores the sql_mode system variable setting in effect when an event is created or altered, and always executes the event with this setting in force, regardless of the current server SQL mode when the event begins executing.

A CREATE EVENT statement that contains an ALTER EVENT statement in its DO clause appears to succeed; however, when the server attempts to execute the resulting scheduled event, the execution fails with an error.

Note

Statements such as SELECT or SHOW that merely return a result set have no effect when used in an event; the output from these is not sent to the MySQL Monitor, nor is it stored anywhere. However, you can use statements such as SELECT ... INTO and INSERT INTO ... SELECT that store a result. (See the next example in this section for an instance of the latter.)

The schema to which an event belongs is the default schema for table references in the DO clause. Any references to tables in other schemas must be qualified with the proper schema name.

As with stored routines, you can use compound-statement syntax in the DO clause by using the BEGIN and END keywords, as shown here:

  1. delimiter |
  2.  
  3. CREATE EVENT e_daily
  4.     ON SCHEDULE
  5.       EVERY 1 DAY
  6.     COMMENT 'Saves total number of sessions then clears the table each day'
  7.     DO
  8.       BEGIN
  9.         INSERT INTO site_activity.totals (time, total)
  10.           SELECT CURRENT_TIMESTAMP, COUNT(*)
  11.             FROM site_activity.sessions;
  12.         DELETE FROM site_activity.sessions;
  13.       END |
  14.  
  15. delimiter ;

This example uses the delimiter command to change the statement delimiter. See Section 24.1, “Defining Stored Programs”.

More complex compound statements, such as those used in stored routines, are possible in an event. This example uses local variables, an error handler, and a flow control construct:

  1. delimiter |
  2.  
  3. CREATE EVENT e
  4.     ON SCHEDULE
  5.       EVERY 5 SECOND
  6.     DO
  7.       BEGIN
  8.         DECLARE v INTEGER;
  9.         DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN END;
  10.  
  11.         SET v = 0;
  12.  
  13.         WHILE v < 5 DO
  14.           INSERT INTO t1 VALUES (0);
  15.           UPDATE t2 SET s1 = s1 + 1;
  16.           SET v = v + 1;
  17.         END WHILE;
  18.     END |
  19.  
  20. delimiter ;

There is no way to pass parameters directly to or from events; however, it is possible to invoke a stored routine with parameters within an event:

  1. CREATE EVENT e_call_myproc
  2.     ON SCHEDULE
  3.       AT CURRENT_TIMESTAMP + INTERVAL 1 DAY
  4.     DO CALL myproc(5, 27);

If an event's definer has privileges sufficient to set global system variables (see Section 5.1.9.1, “System Variable Privileges”), the event can read and write global variables. As granting such privileges entails a potential for abuse, extreme care must be taken in doing so.

Generally, any statements that are valid in stored routines may be used for action statements executed by events. For more information about statements permissible within stored routines, see Section 24.2.1, “Stored Routine Syntax”. You can create an event as part of a stored routine, but an event cannot be created by another event.


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