| 本教程将向您展示如何使用ALTER EVENT语句修改现有的MySQL事件。
在学习完本教程之后，您将了解如何修改事件和计划，如何启用或禁用事件以及如何重命名事件。
MySQL允许您更改现有事件的各种属性。要更改现有事件，请使用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 |
请注意，ALTER EVENT语句仅适用于存在的事件。如果您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。
执行上面查询，得到以下结果 -
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 |
ALTER EVENT示例
我们创建一个示例事件来演示如何使用ALTER EVENT语句的各种功能。
以下语句创建一个事件，每分钟将一条新记录插入到messages表中。
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());
|
改变调度时间
要修改事件为每2分钟运行一次，请使用以下语句：
ALTER
EVENT test_event_04
ON SCHEDULE EVERY 2 MINUTE; |
改变事件的主体代码逻辑
您还可以通过指定新的逻辑来更改事件的主体代码，如下所示：
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; |
上面修改完成后，可以等待2分钟，再次查看messages表：
执行上面查询，得到以下结果 -
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 |
禁用事件
要禁用某个事件,请在ALTER EVENT语句之后使用DISABLE关键字，请使用以下语句：
ALTER
EVENT test_event_04
DISABLE; |
也可以通过使用SHOW EVENTS语句来查看事件的状态，如下所示：
执行上面查询，得到以下结果 -
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 |
启用事件
要启用已禁用的事件，请在ALTER EVENT语句之后使用ENABLE关键字，如下所示：
ALTER
EVENT test_event_04
ENABLE; |
查询上面语句执行结果，得到以下结果 -
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 |
重命名事件
MySQL不提供类似RENAME EVENT语句。幸运的是，我们可以使用ALTER EVENT重命名现有事件，如下所示：
ALTER
EVENT test_event_04
RENAME TO test_event_05; |
查询上面语句执行结果，得到以下结果 -
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 |
将事件移动到其他数据库
可以通过使用RENAME TO子句将事件从一个数据库移动到另一个数据库中，如下所示：
ALTER
EVENT testdb.test_event_05
RENAME TO newdb.test_event_05; |
查询上面语句执行结果，得到以下结果 -
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 |
假设newdb数据库在MySQL数据库服务器中可用。
在本教程中，我们向您展示了如何使用ALTER EVENT语句更改MySQL事件的各种属性。 |
