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！
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user表：
create table user
(
userid integer not null auto_increment primary key,
username varchar(12) not null
)
type=innodb;
password表：
create table password
(
userid integer not null,
password varchar(12) not null,
index (userid),
foreign key (userid) references user (userid)
on delete cascade
on update cascade
)
type=innodb;
1、MySQL支持外键约束，并提供与其它DB相同的功能，但表类型必须为 InnoDB
2、建外键的表的那个列要加上index.