MySQL外键 Cannot add or update a child row错误的实例解释(转)
在MySQL 3.23.44版本后,InnoDB引擎类型的表支持了外键约束。 外键的使用条件: 1.两个表必须是InnoDB表,MyISAM表暂时不支持外键(据说以后的版本有可能支持,但至少目前不支持); 2.外键列必须建立了索引,MySQL 4.1.2以后的版本在建立外键时会自动创建索引,但如果在较早的版本则需要显示建立; 3.外键关系的两个表的列必须是数据类型相似,也就是可 以相互转换类型的列,比如int和tinyint可以,而int和char则不可以; 外键的好处:可以使得两张表关联,保证数据的一致性和实现一些级联操作; 外键的定义语法: 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: 1CREATE TABLE `dage` ( 2 `id` int(11) NOT NULL auto_increment, 3 `name` varchar(32) default '', 4 PRIMARY KEY (`id`) 5) ENGINE=InnoDB DEFAULT CHARSET=latin1; 6 7CREATE TABLE `xiaodi` ( 8 `id` int(11) NOT NULL auto_increment, 9 `dage_id` int(11) default NULL, 10 `name` varchar(32) default '', 11 PRIMARY KEY (`id`), 12 KEY `dage_id` (`dage_id`), 13 CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`) 14) ENGINE=InnoDB DEFAULT CHARSET=latin1; 插入个大哥: 1mysql> insert into dage(name) values('铜锣湾'); 2Query OK, 1 row affected (0.01 sec) 3mysql> select * from dage; 4+----+--------+ 5| id | name | 6+----+--------+ 7| 1 | 铜锣湾 | 8+----+--------+ 91 row in set (0.00 sec) 插入个小弟: 1mysql> insert into xiaodi(dage_id,name) values(1,'铜锣湾_小弟A'); 2Query OK, 1 row affected (0.02 sec) 3 4mysql> select * from xiaodi; 5+----+---------+--------------+ 6| id | dage_id | name | 7+----+---------+--------------+ 8| 1 | 1 | 铜锣湾_小弟A | 9+----+---------+--------------+ 把大哥删除: 1mysql> delete from dage where id=1;
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`))
插入一个新的小弟: 1mysql> 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`)) 3
把外键约束增加事件触发限制: 1mysql> show create table xiaodi; 2 3 CONSTRAINT `xiaodi_ibfk_1` FOREIGN KEY (`dage_id`) REFERENCES `dage` (`id`) 4 5mysql> alter table xiaodi drop foreign key xiaodi_ibfk_1; 6Query OK, 1 row affected (0.04 sec) 7Records: 1 Duplicates: 0 Warnings: 8mysql> alter table xiaodi add foreign key(dage_id) references dage(id) on delete cascade on update cascade; 9Query OK, 1 row affected (0.04 sec) 10Records: 1 Duplicates: 0 Warnings: 0 再次试着把大哥删了: 1mysql> delete from dage where id=1;
2Query OK, 1 row affected (0.01 sec) 3 4mysql> select * from dage; 5Empty set (0.01 sec) 6 7mysql> select * from xiaodi; 8Empty set (0.00 sec)
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!:-)
转自: http://www./wolf/articles/69089.html |
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