在MySQL中利用外键实现级联删除

首先，目前在产品环境可用的MySQL版本（指4.0.x和4.1.x）中，只有InnoDB引擎才允许使用外键，所以，我们的数据表必须使用InnoDB引擎。

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！