MySQL外键 Cannot add or update a child row错误的实例解释
2013-05-17 14:02
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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)c
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
`id`
int
( 11
) NOT
NULL
auto_increment,
3
`name`
varchar
( 32
) default
''
,
4
PRIMARY
KEY
(`id`)
5
) ENGINE
=
InnoDB DEFAULT
CHARSET =
latin1;
6
7
CREATE
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:
1
mysql
11303
>
insert
into
dage(name) values
( '
铜锣湾 '
);
2
Query OK,
1
row affected ( 0.01
sec)
3
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
mysql >
insert
into
xiaodi(dage_id,name) values
( 1
, '
铜锣湾_小弟A '
);
2
Query OK,
1
row affected ( 0.02
sec)
3
4
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
mysql >
delete
from
dage where
id =
1 ;
2
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
mysql >
insert
into
xiaodi(dage_id,name) values
( 2
, '
旺角_小弟A '
);
2
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
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
mysql >
show create
table
xiaodi;
2
3
CONSTRAINT
`xiaodi_ibfk_1` FOREIGN
KEY
(`dage_id`) REFERENCES `dage` (`id`)
4
5
mysql
>
alter
table
xiaodi drop
foreign
key
xiaodi_ibfk_1;
6
Query OK,
1
row affected ( 0.04
sec)
7
Records:
1
Duplicates: 0
Warnings:
8
mysql
>
alter
table
xiaodi add
foreign
key
(dage_id) references
dage(id) on
delete
cascade
on
update
cascade
;
9
Query OK,
1
row affected ( 0.04
sec)
10
Records:
1
Duplicates: 0
Warnings:
0
再次试着把大哥删了:
1
mysql >
delete
from
dage where
id =
1 ;
2
Query OK,
1
row affected ( 0.01
sec)
3
4
mysql
>
select
*
from dage;
5
Empty
set
( 0.01
sec)
6
7
mysql
>
select
*
from xiaodi;
8
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!:-)
转自: http://www.cppblog.com/wolf/articles/69089.html
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)c
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
`id`
int
( 11
) NOT
NULL
auto_increment,
3
`name`
varchar
( 32
) default
''
,
4
PRIMARY
KEY
(`id`)
5
) ENGINE
=
InnoDB DEFAULT
CHARSET =
latin1;
6
7
CREATE
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:
1
mysql
11303
>
insert
into
dage(name) values
( '
铜锣湾 '
);
2
Query OK,
1
row affected ( 0.01
sec)
3
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
mysql >
insert
into
xiaodi(dage_id,name) values
( 1
, '
铜锣湾_小弟A '
);
2
Query OK,
1
row affected ( 0.02
sec)
3
4
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
mysql >
delete
from
dage where
id =
1 ;
2
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
mysql >
insert
into
xiaodi(dage_id,name) values
( 2
, '
旺角_小弟A '
);
2
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
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
mysql >
show create
table
xiaodi;
2
3
CONSTRAINT
`xiaodi_ibfk_1` FOREIGN
KEY
(`dage_id`) REFERENCES `dage` (`id`)
4
5
mysql
>
alter
table
xiaodi drop
foreign
key
xiaodi_ibfk_1;
6
Query OK,
1
row affected ( 0.04
sec)
7
Records:
1
Duplicates: 0
Warnings:
8
mysql
>
alter
table
xiaodi add
foreign
key
(dage_id) references
dage(id) on
delete
cascade
on
update
cascade
;
9
Query OK,
1
row affected ( 0.04
sec)
10
Records:
1
Duplicates: 0
Warnings:
0
再次试着把大哥删了:
1
mysql >
delete
from
dage where
id =
1 ;
2
Query OK,
1
row affected ( 0.01
sec)
3
4
mysql
>
select
*
from dage;
5
Empty
set
( 0.01
sec)
6
7
mysql
>
select
*
from xiaodi;
8
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!:-)
转自: http://www.cppblog.com/wolf/articles/69089.html
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