在MySQL中利用外键实现级联删除！

<转自>http://www.chinaunix.net/jh/17/462977.html

下面，我们先创建以下测试用数据库表：


CREATE TABLE `roottb` (

  `id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,

  `data` VARCHAR(100) NOT NULL DEFAULT '',

  PRIMARY KEY (`id`)

) TYPE=InnoDB;



CREATE TABLE `subtb` (

  `id` INT(11) UNSIGNED AUTO_INCREMENT NOT NULL,

  `rootid` INT(11) UNSIGNED NOT NULL DEFAULT '0',

  `data` VARCHAR(100) NOT NULL DEFAULT '',

  PRIMARY KEY (`id`),

  INDEX (`rootid`),

  FOREIGN KEY (`rootid`) REFERENCES roottb(`id`) ON DELETE CASCADE

) TYPE=InnoDB;



注意：
1、必须使用InnoDB引擎；
2、外键必须建立索引（INDEX）；
3、外键绑定关系这里使用了“ ON DELETE CASCADE”，意思是如果外键对应数据被删除，将关联数据完全删除，更多信息请参考MySQL手册中关于InnoDB的文档；

好，接着我们再来插入测试数据：


INSERT INTO `roottb` (`id`,`data`)

  VALUES ('1', 'test root line 1'),

         ('2', 'test root line 2'),

         ('3', 'test root line 3');



INSERT INTO `subtb` (`id`,`rootid`,`data`)

  VALUES ('1', '1', 'test sub line 1 for root 1'),

         ('2', '1', 'test sub line 2 for root 1'),

         ('3', '1', 'test sub line 3 for root 1'),

         ('4', '2', 'test sub line 1 for root 2'),

         ('5', '2', 'test sub line 2 for root 2'),

         ('6', '2', 'test sub line 3 for root 2'),

         ('7', '3', 'test sub line 1 for root 3'),

         ('8', '3', 'test sub line 2 for root 3'),

         ('9', '3', 'test sub line 3 for root 3');

  

我们先看一下当前数据表的状态：


mysql>; show tables;

+----------------+

| Tables_in_test |

+----------------+

| roottb         |

| subtb          |

+----------------+

2 rows in set (0.00 sec)



mysql>; select * from `roottb`;

+----+------------------+

| id | data             |

+----+------------------+

|  1 | test root line 1 |

|  2 | test root line 2 |

|  3 | test root line 3 |

+----+------------------+

3 rows in set (0.05 sec)



mysql>; select * from `subtb`;

+----+--------+----------------------------+

| id | rootid | data                       |

+----+--------+----------------------------+

|  1 |      1 | test sub line 1 for root 1 |

|  2 |      1 | test sub line 2 for root 1 |

|  3 |      1 | test sub line 3 for root 1 |

|  4 |      2 | test sub line 1 for root 2 |

|  5 |      2 | test sub line 2 for root 2 |

|  6 |      2 | test sub line 3 for root 2 |

|  7 |      3 | test sub line 1 for root 3 |

|  8 |      3 | test sub line 2 for root 3 |

|  9 |      3 | test sub line 3 for root 3 |

+----+--------+----------------------------+

9 rows in set (0.01 sec)



嗯，一切都正常，好，下面我们要试验我们的级联删除功能了。

我们将只删除roottb表中id为2的数据记录，看看subtb表中rootid为2的相关子纪录是否会自动删除：


mysql>; delete from `roottb` where `id`='2';

Query OK, 1 row affected (0.03 sec)



mysql>; select * from `roottb`;

+----+------------------+

| id | data             |

+----+------------------+

|  1 | test root line 1 |

|  3 | test root line 3 |

+----+------------------+

2 rows in set (0.00 sec)



mysql>; select * from `subtb`;

+----+--------+----------------------------+

| id | rootid | data                       |

+----+--------+----------------------------+

|  1 |      1 | test sub line 1 for root 1 |

|  2 |      1 | test sub line 2 for root 1 |

|  3 |      1 | test sub line 3 for root 1 |

|  7 |      3 | test sub line 1 for root 3 |

|  8 |      3 | test sub line 2 for root 3 |

|  9 |      3 | test sub line 3 for root 3 |

+----+--------+----------------------------+

6 rows in set (0.01 sec)



嗯，看subtb表中对应数据确实自动删除了，测试成功。

结论：在MySQL中利用外键实现级联删除成功！