时间:2021-07-01 10:21:17 帮助过:15人阅读
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:
1 1 CREATE TABLE `dage` ( 2 2 `id` int(11) NOT NULL auto_increment, 3 3 `name` varchar(32) default ‘‘, 4 4 PRIMARY KEY (`id`) 5 5) ENGINE=InnoDB DEFAULT CHARSET=latin1; 6 6 7 7CREATE TABLE `xiaodi` ( 8 8 `id` int(11) NOT NULL auto_increment, 9 9 `dage_id` int(11) default NULL, 10 10 `name` varchar(32) default ‘‘, 11 11 PRIMARY KEY (`id`), 12 12 KEY `dage_id` (`dage_id`), 13 13 CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`) 14 14) ENGINE=InnoDB DEFAULT CHARSET=latin1;
插入个大哥:
1 1mysql> insert into dage(name) values(‘铜锣湾‘); 2 2Query OK, 1 row affected (0.01 sec) 3 3mysql> select * from dage; 4 4+----+--------+ 5 5| id | name | 6 6+----+--------+ 7 7| 1 | 铜锣湾 | 8 8+----+--------+ 9 91 row in set (0.00 sec)
插入个小弟:
1 1mysql> insert into xiaodi(dage_id,name) values(1,‘铜锣湾_小弟A‘); 2 2Query OK, 1 row affected (0.02 sec) 3 3 4 4mysql> select * from xiaodi; 5 5+----+---------+--------------+ 6 6| id | dage_id | name | 7 7+----+---------+--------------+ 8 8| 1 | 1 | 铜锣湾_小弟A | 9 9+----+---------+--------------+
把大哥删除:
1 1mysql> delete from dage where id=1; 2 2ERROR 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:
1 1mysql> insert into xiaodi(dage_id,name) values(2,‘旺角_小弟A‘); 2 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`)) 3
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:
1 1mysql> show create table xiaodi; 2 2 3 3 CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`) 4 4 5 5mysql> alter table xiaodi drop foreign key xiaodi_ibfk_1; 6 6Query OK, 1 row affected (0.04 sec) 7 7Records: 1 Duplicates: 0 Warnings: 8 8mysql> alter table xiaodi add foreign key(dage_id) references dage(id) on delete cascade on update cascade; 9 9Query OK, 1 row affected (0.04 sec) 10 10Records: 1 Duplicates: 0 Warnings: 0
再次试着把大哥删了:
1 1mysql> delete from dage where id=1; 2 2Query OK, 1 row affected (0.01 sec) 3 3 4 4mysql> select * from dage; 5 5Empty set (0.01 sec) 6 6 7 7mysql> select * from xiaodi; 8 8Empty 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!:-)
MYSQL外键(Foreign Key)的使用
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