本教學將向您展示如何使用ALTER EVENT語句修改現有的MySQL事件。 在學習完本教學之後，您將了解如何修改事件和計劃，如何啟用或禁用事件以及如何重新命名事件。
MySQL允許您更改現有事件的各種屬性。要更改現有事件，請使用ALTER EVENT語句，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
請注意，ALTER EVENT語句僅適用於存在的事件。如果您嘗試修改不存在的事件，MySQL將會發出一條錯誤訊息，因此在更改事件之前，應先使用SHOW EVENTS語句檢查事件的存在。
SHOW EVENTS FROM testdb;
執行上面查詢，得到以下結果 -
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
我們建立一個範例事件來演示如何使用ALTER EVENT語句的各種功能。
以下語句建立一個事件，每分鐘將一條新記錄插入到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());
改變排程時間
要修改事件為每2分鐘執行一次，請使用以下語句：
ALTER EVENT test_event_04
ON SCHEDULE EVERY 2 MINUTE;
改變事件的主體程式碼邏輯
您還可以通過指定新的邏輯來更改事件的主體程式碼，如下所示：
ALTER EVENT test_event_04
DO
INSERT INTO messages(message,created_at)
VALUES('Message from event',NOW());
-- 清空表中的資料
truncate messages;
上面修改完成後，可以等待2分鐘，再次檢視messages表：
SELECT * FROM messages;
執行上面查詢，得到以下結果 -
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
要禁用某個事件,請在ALTER EVENT語句之後使用DISABLE關鍵字，請使用以下語句：
ALTER EVENT test_event_04
DISABLE;
也可以通過使用SHOW EVENTS語句來檢視事件的狀態，如下所示：
SHOW EVENTS FROM testdb;
執行上面查詢，得到以下結果 -
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
啟用事件
要啟用已禁用的事件，請在ALTER EVENT語句之後使用ENABLE關鍵字，如下所示：
ALTER EVENT test_event_04
ENABLE;
查詢上面語句執行結果，得到以下結果 -
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
重新命名事件
MySQL不提供類似RENAME EVENT語句。幸運的是，我們可以使用ALTER EVENT重新命名現有事件，如下所示：
ALTER EVENT test_event_04
RENAME TO test_event_05;
查詢上面語句執行結果，得到以下結果 -
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
將事件移動到其他資料庫
可以通過使用RENAME TO子句將事件從一個資料庫移動到另一個資料庫中，如下所示：
ALTER EVENT testdb.test_event_05
RENAME TO newdb.test_event_05;
查詢上面語句執行結果，得到以下結果 -
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
假設newdb資料庫在MySQL資料庫伺服器中可用。
在本教學中，我們向您展示了如何使用ALTER EVENT語句更改MySQL事件的各種屬性。