在MySQL中利用外键实现级联删除

作者：Dirk (dirk.ye AT gmail.com)

日期：2004/12/08

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。

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；

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：

show tables;
----------------
| Tables_in_test |
----------------
| roottb |
| subtb |
----------------
2 rows in set (0.00 sec)

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)

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：

delete from `roottb` where `id`='2';
Query OK, 1 row affected (0.03 sec)

select * from `roottb`;
---- ------------------
| id | data |
---- ------------------
| 1 | test root line 1 |
| 3 | test root line 3 |
---- ------------------
2 rows in set (0.00 sec)

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！