我们如何修改现有的 MySQL 事件？

借助 ALTER EVENT 语句，我们可以修改现有的 MySQL 事件。我们可以更改事件的各种属性。ALTER EVENT 的语法如下：

   ALTER EVENT event_name
    ON SCHEDULE schedule
ON COMPLETION [NOT] PRESERVE
  RENAME TO new_event_name
    ENABLE | DISABLE
           DO
       event_body

为了理解它，我们将举例说明如下 -

示例

假设我们有一个事件，如下 -

mysql> Create event hello ON SCHEDULE EVERY 1 Minute DO INSERT INTO event_messages(message, generated_at) Values ('Alter event testing', 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 |
|  3 | Alter event testing | 2017-11-22 21:08:37 |
+----+---------------------+---------------------+
3 rows in set (0.00 sec)

mysql> ALTER EVENT hello ON SCHEDULE EVERY 2 MINUTE;
Query OK, 0 rows affected (0.00 sec)

上述查询会将事件的时间表从 1 分钟更改为 2 分钟。下面的查询将更改事件的正文。

mysql> ALTER EVENT hello DO INSERT INTO event_messages(message,generated_at) VALUES('ALTERED',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 |
|  3 | Alter event testing | 2017-11-22 21:08:37 |
|  4 | Alter event testing | 2017-11-22 21:09:15 |
|  5 | ALTERED             | 2017-11-22 21:11:15 |
+----+---------------------+---------------------+
5 rows in set (0.00 sec)

上面的结果集显示我们在2分钟后收到了更改的消息。

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