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每天一个Linux命令之shell单引号和双引号的经典解释

开发者 https://www.devze.com 2020-01-13 15:54 出处:网络 作者: walker86
这篇文章主要给大家介绍了关于每天一个Linux命令之shell单引号和双引号的经典解释,文中通过示例代码介绍的非常详细,对大家学习或者使用Linux系统具有一定的参考学习价值,需要的朋友们下面来一起学习学习吧

本文实例讲述了mysql事件之修改事件(ALTER EVENT)、禁用事件(DISABLE)、启用事件(ENABLE)、事件重命名及数据库事件迁移操作。分享给大家供大家参考,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

好啦,本次记录就到这里了。

更多关于MySQL相关内容感兴趣的读者可查看本站专题:《MySQL索引操作技巧汇总》、《MySQL常用函数大汇总》、《MySQL日志操作技巧大全》、《MySQL事务操作技巧汇总》、《MySQL存储过程技巧大全》及《MySQL数据库锁相关技巧汇总》

希望本文所述对大家MySQL数据库计有所帮助。

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