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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"htmlcode">

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!:-)

标签:
MySQL,主键,外键,教程,MySQL教程

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