27.4.1 Event Scheduler Overview

MySQL Events are tasks that run according to a schedule. Therefore, we sometimes refer to them as scheduled events. When you create an event, you are creating a named database object containing one or more SQL statements to be executed at one or more regular intervals, beginning and ending at a specific date and time. Conceptually, this is similar to the idea of the Unix crontab (also known as a “cron job”) or the Windows Task Scheduler.

Scheduled tasks of this type are also sometimes known as “temporal triggers”, implying that these are objects that are triggered by the passage of time. While this is essentially correct, we prefer to use the term events to avoid confusion with triggers of the type discussed in Section 27.3, “Using Triggers”. Events should more specifically not be confused with “temporary triggers”. Whereas a trigger is a database object whose statements are executed in response to a specific type of event that occurs on a given table, a (scheduled) event is an object whose statements are executed in response to the passage of a specified time interval.

While there is no provision in the SQL Standard for event scheduling, there are precedents in other database systems, and you may notice some similarities between these implementations and that found in the MySQL Server.

MySQL Events have the following major features and properties:

• In MySQL, an event is uniquely identified by its name and the schema to which it is assigned.

• An event performs a specific action according to a schedule. This action consists of an SQL statement, which can be a compound statement in a BEGIN ... END block if desired (see Section 15.6, “Compound Statement Syntax”). An event's timing can be either one-time or recurrent. A one-time event executes one time only. A recurrent event repeats its action at a regular interval, and the schedule for a recurring event can be assigned a specific start day and time, end day and time, both, or neither. (By default, a recurring event's schedule begins as soon as it is created, and continues indefinitely, until it is disabled or dropped.)

  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.

• Users can create, modify, and drop scheduled events using SQL statements intended for these purposes. Syntactically invalid event creation and modification statements fail with an appropriate error message. A user may include statements in an event's action which require privileges that the user does not actually have. The event creation or modification statement succeeds but the event's action fails. See Section 27.4.6, “The Event Scheduler and MySQL Privileges” for details.

• Many of the properties of an event can be set or modified using SQL statements. These properties include the event's name, timing, persistence (that is, whether it is preserved following the expiration of its schedule), status (enabled or disabled), action to be performed, and the schema to which it is assigned. See Section 15.1.3, “ALTER EVENT Statement”.

  The default definer of an event is the user who created the event, unless the event has been altered, in which case the definer is the user who issued the last ALTER EVENT statement affecting that event. An event can be modified by any user having the EVENT privilege on the database for which the event is defined. See Section 27.4.6, “The Event Scheduler and MySQL Privileges”.

• An event's action statement may include most SQL statements permitted within stored routines. For restrictions, see Section 27.8, “Restrictions on Stored Programs”.