MYSQL外键(Foreign Key)的使用（二）

作者：Dirk (dirk.ye AT gmail.com)
Url：http://dirk.pdx.cn
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。
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！