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:
[CONSTRAINT symbol] FOREIGN KEY [id] (index_col_name, ...)
REFERENCES tbl_name (index_col_name, ...)
[ON DELETE {RESTRICT | CASCADE
| SET NULL | NO ACTION | SET DEFAULT}]
[ON UPDATE {RESTRICT | CASCADE |
SET NULL | NO ACTION | SET DEFAULT}]
该语法可以在 CREATE TABLE 和 ALTER TABLE
时使用,如果不指定CONSTRAINT symbol,MYSQL会自动生成一个名字。
ON DELETE、ON
UPDATE表示事件触发限制,可设参数:
RESTRICT(限制外表中的外键改动)
CASCADE(跟随外键改动)
SET
NULL(设空值)
SET DEFAULT(设默认值)
NO ACTION(无动作,默认的)
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
CREATE TABLE `dage` (
2![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
`id`
int(
11)
NOT NULL auto_increment,
3![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
`name`
varchar(
32)
default '',
4
PRIMARY KEY (`id`)
5![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
) ENGINE
=InnoDB
DEFAULT CHARSET
=latin1;
6
7
CREATE TABLE `xiaodi` (
8![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
`id`
int(
11)
NOT NULL auto_increment,
9![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
`dage_id`
int(
11)
default NULL,
10![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
`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![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
) ENGINE
=InnoDB
DEFAULT CHARSET
=latin1;
插入个大哥:
1![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> insert into dage(name)
values(
'铜锣湾');
2![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Query OK,
1 row affected (
0.01 sec)
3![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> select * from dage;
4
+----+--------+
5
| id
| name
|6
+----+--------+
7
| 1 | 铜锣湾
|8
+----+--------+
9
1 row
in set (
0.00 sec)
插入个小弟:
1![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> insert into xiaodi(dage_id,name)
values(
1,
'铜锣湾_小弟A');
2![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Query OK,
1 row affected (
0.02 sec)
3
4![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> 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:
1![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> delete from dage
where id
=1;
2![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
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:
1![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> insert into xiaodi(dage_id,name)
values(
2,
'旺角_小弟A');
2![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
ERROR
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![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
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![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> show
create table xiaodi;
2![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
![](//www.cppblog.com/Images/dot.gif)
3
CONSTRAINT `xiaodi_ibfk_1`
FOREIGN KEY (`dage_id`)
REFERENCES `dage` (`id`)
4![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
![](//www.cppblog.com/Images/dot.gif)
5![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> alter table xiaodi
drop foreign key xiaodi_ibfk_1;
6![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Query OK,
1 row affected (
0.04 sec)
7![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Records:
1 Duplicates:
0 Warnings:
8![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> alter table xiaodi
add foreign key(dage_id)
references dage(id)
on delete cascade on update cascade;
9![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Query OK,
1 row affected (
0.04 sec)
10![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Records:
1 Duplicates:
0 Warnings:
0
再次试着把大哥删了:
1![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> delete from dage
where id
=1;
2![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Query OK,
1 row affected (
0.01 sec)
3
4![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> select * from dage;
5![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
Empty
set (
0.01 sec)
6
7![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
mysql
> select * from xiaodi;
8![](//www.cppblog.com/Images/OutliningIndicators/None.gif)
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!:-)