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具体如下:
我们要知道,MySQL允许我们更改现有事件的各种属性。如果我们要更改现有事件,可以使用ALTER EVENT语句,如下所示:
ALTER EVENT event_name ON SCHEDULE schedule ON COMPLETION [NOT] PRESERVE RENAME TO new_event_name ENABLE | DISABLE DO event_body
ALTER EVENT语句仅适用于存在的事件,如果我们尝试修改不存在的事件,MySQL将会发出一条错误消息,因此在更改事件之前,应先使用SHOW EVENTS语句检查事件的存在:
mysql> SHOW EVENTS FROM testdb; +--------+---------------+----------------+-----------+----------+---------------------+----------------+----------------+--------+------+----------+------------+----------------------+----------------------+--------------------+ | Db | Name | Definer | Time zone | Type | Execute at | Interval value | Interval field | Starts | Ends | Status | Originator | character_set_client | collation_connection | Database Collation | +--------+---------------+----------------+-----------+----------+---------------------+----------------+----------------+--------+------+----------+------------+----------------------+----------------------+--------------------+ | testdb | test_event_02 | root@localhost | SYSTEM | ONE TIME | 2017-08-03 04:24:48 | NULL | NULL | NULL | NULL | DISABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | +--------+---------------+----------------+-----------+----------+---------------------+----------------+----------------+--------+------+----------+------------+----------------------+----------------------+--------------------+ 1 row in set
创建一个每分钟将一条新记录插入到messages表中的示例事件来演示如何使用ALTER EVENT语句的各种功能:
USE testdb; CREATE EVENT test_event_04 ON SCHEDULE EVERY 1 MINUTE DO INSERT INTO messages(message,created_at) VALUES('Test ALTER EVENT statement',NOW());
我们来把事件修改为为每2分钟运行一次:
ALTER EVENT test_event_04 ON SCHEDULE EVERY 2 MINUTE;
我们还可以通过指定新的逻辑来更改事件的主体代码:
ALTER EVENT test_event_04 DO INSERT INTO messages(message,created_at) VALUES('Message from event',NOW()); -- 清空表中的数据 truncate messages;
修改完成后,可以等待2分钟,再次查看messages表:
mysql> SELECT * FROM messages; +----+--------------------+---------------------+ | id | message | created_at | +----+--------------------+---------------------+ | 1 | Message from event | 2017-08-03 04:46:47 | | 2 | Message from event | 2017-08-03 04:48:47 | +----+--------------------+---------------------+ 2 rows in set
我们可以在ALTER EVENT语句之后使用DISABLE关键字来禁用某个事件:
ALTER EVENT test_event_04 DISABLE;
我们也可以通过使用SHOW EVENTS语句来查看事件的状态:
mysql> SHOW EVENTS FROM testdb; +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ | Db | Name | Definer | Time zone | Type | Execute at | Interval value | Interval field | Starts | Ends | Status | Originator | character_set_client | collation_connection | Database Collation | +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ | testdb | test_event_02 | root@localhost | SYSTEM | ONE TIME | 2017-08-03 04:24:48 | NULL | NULL | NULL | NULL | DISABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | | testdb | test_event_04 | root@localhost | SYSTEM | RECURRING | NULL | 2 | MINUTE | 2017-08-03 04:44:47 | NULL | DISABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ 2 rows in set
我们可以在ALTER EVENT语句之后使用ENABLE关键字来启用事件:
ALTER EVENT test_event_04 ENABLE;
查看下事件状态:
mysql> SHOW EVENTS FROM testdb; +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ | Db | Name | Definer | Time zone | Type | Execute at | Interval value | Interval field | Starts | Ends | Status | Originator | character_set_client | collation_connection | Database Collation | +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ | testdb | test_event_02 | root@localhost | SYSTEM | ONE TIME | 2017-08-03 04:24:48 | NULL | NULL | NULL | NULL | DISABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | | testdb | test_event_04 | root@localhost | SYSTEM | RECURRING | NULL | 2 | MINUTE | 2017-08-03 04:44:47 | NULL | ENABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ 2 rows in set
我们再来尝试使用ALTER EVENT重命名现有事件:
ALTER EVENT test_event_04 RENAME TO test_event_05;
来查看下事件状态:
mysql> SHOW EVENTS FROM testdb; +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ | Db | Name | Definer | Time zone | Type | Execute at | Interval value | Interval field | Starts | Ends | Status | Originator | character_set_client | collation_connection | Database Collation | +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ | testdb | test_event_02 | root@localhost | SYSTEM | ONE TIME | 2017-08-03 04:24:48 | NULL | NULL | NULL | NULL | DISABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | | testdb | test_event_05 | root@localhost | SYSTEM | RECURRING | NULL | 2 | MINUTE | 2017-08-03 04:44:47 | NULL | ENABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | +--------+---------------+----------------+-----------+-----------+---------------------+----------------+----------------+---------------------+------+----------+------------+----------------------+----------------------+--------------------+ 2 rows in set
完事再来通过使用RENAME TO子句将事件从一个数据库移动到另一个数据库中:
ALTER EVENT testdb.test_event_05 RENAME TO newdb.test_event_05;
再来查看事件状态:
mysql> SHOW EVENTS FROM newdb; +-------+---------------+----------------+-----------+-----------+------------+----------------+----------------+---------------------+------+---------+------------+----------------------+----------------------+--------------------+ | Db | Name | Definer | Time zone | Type | Execute at | Interval value | Interval field | Starts | Ends | Status | Originator | character_set_client | collation_connection | Database Collation | +-------+---------------+----------------+-----------+-----------+------------+----------------+----------------+---------------------+------+---------+------------+----------------------+----------------------+--------------------+ | newdb | test_event_05 | root@localhost | SYSTEM | RECURRING | NULL | 2 | MINUTE | 2017-08-03 04:44:47 | NULL | ENABLED | 0 | utf8 | utf8_general_ci | utf8_general_ci | +-------+---------------+----------------+-----------+-----------+------------+----------------+----------------+---------------------+------+---------+------------+----------------------+----------------------+--------------------+ 1 row in set
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