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
(also known as a “cron job”) or the Windows Task
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 20.3, “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 5.5, 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
END block if desired (see
Section 13.6, “MySQL 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
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 20.4.6, “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 13.1.2, “ALTER EVENT Syntax”.
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 20.4.6, “The Event Scheduler and MySQL Privileges”.
An event's action statement may include most SQL statements permitted within stored routines. For restrictions, see Section D.1, “Restrictions on Stored Programs”.