主键与外键的关系,通俗点儿讲,我现在有一个论坛,有两张表,一张是主贴 thread,一张是回帖 reply
先说说主键,主键是表里面唯一识别记录的字段,一般是帖子id,体现在访问的时候,例如是
thread.php?id=1 表示我要访问的是帖子id是1 的帖子~
再来说说外键,当我们删除某个帖子的时候,需要执行另一个操作,就是删除所有回帖,如果正常情况下,我们需要执行两次delete操作(thread和 reply),这时候如果存在外键,例如,在reply 表里面建立一个指向thread表的主键(id)的外键(这个外键绑的字段,必须是对应帖子的id),并指定响应 delete ,那你在删除 thread 的时候,mysql 自己会帮你把 reply 表中这个帖子的回复都删掉,而不需要你手动再去执行一次reply表的delete操作~
至于两者之间的关系,在刚才的例子中,reply 表的外键,指向的就是 thread 表的主键~~
搞个例子,简单演示一下使用,做dage和xiaodi两个表,大哥表是主键,小弟表是外键:
建表:
CREATE TABLE `dage` ( `id` int(11) NOT NULL auto_increment, `name` varchar(32) default '', PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1; CREATE TABLE `xiaodi` ( `id` int(11) NOT NULL auto_increment, `dage_id` int(11) default NULL, `name` varchar(32) default '', PRIMARY KEY (`id`), KEY `dage_id` (`dage_id`), CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
插入个大哥:
mysql> insert into dage(name) values('铜锣湾');
Query OK, 1 row affected (0.01 sec)
mysql> select * from dage;
+----+--------+ | id | name | +----+--------+ | 1 | 铜锣湾 | +----+--------+ 1 row in set (0.00 sec)
插入个小弟:
mysql> insert into xiaodi(dage_id,name) values(1,'铜锣湾_小弟A');
Query OK, 1 row affected (0.02 sec)
mysql> select * from xiaodi;
+----+---------+--------------+ | id | dage_id | name | +----+---------+--------------+ | 1 | 1 | 铜锣湾_小弟A | +----+---------+--------------+
把大哥删除:
mysql> delete from dage where id=1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`bstar/xiaodi`, CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`))
提示:不行呀,有约束的,大哥下面还有小弟,可不能扔下我们不管呀!
插入一个新的小弟:
mysql> insert into xiaodi(dage_id,name) values(2,'旺角_小弟A');
2ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`bstar/xiaodi`, CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`))
提示:小子,想造反呀!你还没大哥呢!
把外键约束增加事件触发限制:
mysql> show create table xiaodi;CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`)
mysql> alter table xiaodi drop foreign key xiaodi_ibfk_1;
Query OK, 1 row affected (0.04 sec) Records: 1 Duplicates: 0 Warnings:
mysql> alter table xiaodi add foreign key(dage_id) references dage(id) on delete cascade on update cascade;
Query OK, 1 row affected (0.04 sec) Records: 1 Duplicates: 0 Warnings: 0
再次试着把大哥删了:
mysql> delete from dage where id=1;
Query OK, 1 row affected (0.01 sec)
mysql> select * from dage;
Empty set (0.01 sec)
mysql> select * from xiaodi;
Empty set (0.00 sec)
得,这回对应的小弟也没了,没办法,谁让你跟我on delete cascade了呢!
例子说明的应该蛮清楚了吧,其他功能对应手册自己实践吧!:-)