我们知道，事件过期后会自动删除，并且我们无法从 SHOW EVENTS 语句中看到它。要改变这种行为，我们可以在创建事件时使用 ON COMPLETION PRESERVE。可以通过以下示例来理解 –
示例
mysql> Create table event_messages(ID INT NOT NULL PRIMARY KEY AUTO_INCREMENT, MESSAGE VARCHAR(255) NOT NULL, Generated_at DATETIME NOT NULL);Query OK, 0 rows affected (0.61 sec)
以下查询将在不使用 ON COMPLETION PRESERVE 的情况下创建事件，因此在 SHOW EVENTS FROM db_name 查询的输出中不会看到该事件。
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mysql> CREATE EVENT testing_event_without_Preserves ON SCHEDULE AT CURRENT_TIMESTAMP DO INSERT INTO event_messages(message,generated_at) Values('Without Preserve',NOW());Query OK, 0 rows affected (0.00 sec)mysql> Select * from event_messages;+----+------------------+---------------------+| ID | MESSAGE | Generated_at |+----+------------------+---------------------+| 1 | Without Preserve | 2017-11-22 20:32:13 |+----+------------------+---------------------+1 row in set (0.00 sec)mysql> SHOW EVENTS FROM queryG*************************** 1. row *************************** Db: query Name: testing_event5 Definer: root@localhost Time zone: SYSTEM Type: ONE TIME Execute at: 2017-11-22 17:09:11 Interval value: NULL Interval field: NULL Starts: NULL Ends: NULL Status: DISABLED Originator: 0character_set_client: cp850collation_connection: cp850_general_ci Database Collation: latin1_swedish_ci1 row in set (0.00 sec)
以下查询将使用 ON COMPLETION PRESERVE 创建一个事件，因此可以在 SHOW EVENTS FROM db_name 查询的输出中看到该事件。
mysql> CREATE EVENT testing_event_with_Preserves ON SCHEDULE AT CURRENT_TIMESTAMP ON COMPLETION PRESERVE DO INSERT INTO event_messages(message,generated_at) Values('With Preserve',NOW());Query OK, 0 rows affected (0.00 sec)mysql> Select * from event_messages;+----+------------------+---------------------+| ID | MESSAGE | Generated_at |+----+------------------+---------------------+| 1 | Without Preserve | 2017-11-22 20:32:13 || 2 | With Preserve | 2017-11-22 20:35:12 |+----+------------------+---------------------+2 rows in set (0.00 sec)mysql> SHOW EVENTS FROM queryG*************************** 1. row *************************** Db: query Name: testing_event5 Definer: root@localhost Time zone: SYSTEM Type: ONE TIME Execute at: 2017-11-22 17:09:11 Interval value: NULL Interval field: NULL Starts: NULL Ends: NULL Status: DISABLED Originator: 0character_set_client: cp850collation_connection: cp850_general_ci Database Collation: latin1_swedish_ci*************************** 2. row *************************** Db: query Name: testing_event_with_Preserves Definer: root@localhost Time zone: SYSTEM Type: ONE TIME Execute at: 2017-11-22 20:35:12 Interval value: NULL Interval field: NULL Starts: NULL Ends: NULL Status: DISABLED Originator: 0character_set_client: cp850collation_connection: cp850_general_ci Database Collation: latin1_swedish_ci2 rows in set (0.00 sec)
以上就是创建事件时 ON COMPLETION PRESERVE 子句有什么用？的详细内容，更多请关注创想鸟其它相关文章！
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