mysql foreign key

mysql foreign key（即外键）的作用是为了保证数据的一致性。具体看下测试效果：

1、创建一个表TStudent，记录学生的信息；

mysql> create table TStudent (ID int not null auto_increment,name varchar(20),addr varchar(20),primary key(ID));
Query OK, 0 rows affected (0.09 sec)

mysql> desc TStudent;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| ID    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20) | YES  |     | NULL    |                |
| addr  | varchar(20) | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

2、再建一表TGrade，记录学生成绩，该表ID列为外键，关联TStudent 的ID列；

mysql> create table TGrade (GID int null auto_increment,Chinese float(5,2),English float(5,2),ID int ,<span style="color:#ff0000;">foreign key(ID) references TStudent(ID)</span>,primary key(GID));
Query OK, 0 rows affected (0.08 sec)

mysql> desc TGrade;
+---------+------------+------+-----+---------+----------------+
| Field   | Type       | Null | Key | Default | Extra          |
+---------+------------+------+-----+---------+----------------+
| GID     | int(11)    | NO   | PRI | NULL    | auto_increment |
| Chinese | float(5,2) | YES  |     | NULL    |                |
| English | float(5,2) | YES  |     | NULL    |                |
| ID      | int(11)    | YES  | MUL | NULL    |                |
+---------+------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

3、往表TGrade先添加数据。

mysql> insert into TStudent (name ,addr) values('zhangsan','sichuang'),('lisi','hunan'),('wangwu','zhejiang');
Query OK, 3 rows affected (0.05 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from TStudent;
+----+----------+----------+
| ID | name     | addr     |
+----+----------+----------+
|  1 | zhangsan | sichuang |
|  2 | lisi     | hunan    |
|  3 | wangwu   | zhejiang |
+----+----------+----------+
3 rows in set (0.00 sec)

4、尝试向TGrade表中添加数据，数据中的ID（5）在TStudent中不存在时，报错。

mysql> insert into TGrade (Chinese,English,ID) values(95,90,5);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`mydb1`.`TGrade`, CONSTRAINT `TGrade_ibfk_1` FOREIGN KEY (`ID`) REFERENCES `TStudent` (`ID`))

5、尝试向TGrade表中添加数据，数据中的ID（1）在TStudent中存在时，可成功添加。

mysql> insert into TGrade (Chinese,English,ID) values(95,90,1);
Query OK, 1 row affected (0.04 sec)

mysql> select * from TGrade;
+-----+---------+---------+------+
| GID | Chinese | English | ID   |
+-----+---------+---------+------+
|   2 |   95.00 |   90.00 |    1 |
+-----+---------+---------+------+
1 row in set (0.00 sec)

6、尝试删除TStudent中的数据，该数据的ID被TGrade使用时，删除失败，报错。

mysql> delete from TStudent where name='zhangsan';
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`mydb1`.`TGrade`, CONSTRAINT `TGrade_ibfk_1` FOREIGN KEY (`ID`) REFERENCES `TStudent` (`ID`))

7、尝试更新TStudent中的ID，该ID被TGrade使用时，更新失败，报错。

mysql> update TStudent set ID=10 where name='zhangsan';
ERROR 1451 (
4000
23000): Cannot delete or update a parent row: a foreign key constraint fails (`mydb1`.`TGrade`, CONSTRAINT `TGrade_ibfk_1` FOREIGN KEY (`ID`) REFERENCES `TStudent` (`ID`))

8、如果先删除TGrade中的ID，再删除TSudent中该ID的记录，可删除成功。

mysql> select * from TGrade;
+-----+---------+---------+------+
| GID | Chinese | English | ID   |
+-----+---------+---------+------+
|   2 |   95.00 |   90.00 |    1 |
+-----+---------+---------+------+
1 row in set (0.00 sec)

mysql> delete from TGrade where ID=1;
Query OK, 1 row affected (0.04 sec)

mysql>  delete from TStudent where name='zhangsan';
Query OK, 1 row affected (0.05 sec)

mysql> select * from TStudent;
+----+--------+----------+
| ID | name   | addr     |
+----+--------+----------+
|  2 | lisi   | hunan    |
|  3 | wangwu | zhejiang |
+----+--------+----------+
2 rows in set (0.00 sec)

9、给TGrade ID列增加 on delete cascade on update cascade
参数；

1）先去掉foreign属性

mysql> alter table TGrade drop foreign key TGrade_ibfk_1;
Query OK, 0 rows affected (0.20 sec)
Records: 0 Duplicates: 0 Warnings: 0
2）再加on delete cascade on update cascade属性

mysql> alter table TGrade add foreign key(ID) references TStudent(ID) on delete cascade on update cascade;
Query OK, 0 rows affected (0.24 sec)
Records: 0  Duplicates: 0  Warnings: 0

3）往TGrade中加入数据用来做删除测试。
mysql> insert into TGrade (Chinese ,English,ID) values(90,93,2),(91,93,3);
Query OK, 2 rows affected (0.03 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from TGrade;
+-----+---------+---------+------+
| GID | Chinese | English | ID   |
+-----+---------+---------+------+
|   4 |   90.00 |   93.00 |    2 |
|   5 |   91.00 |   93.00 |    3 |
+-----+---------+---------+------+
2 rows in set (0.00 sec)


4）从TStudent ID中删除ID为2的数据，发现TGrade表中ID为2为的记录也删除了。

mysql> delete from TStudent where name='lisi';
Query OK, 1 row affected (0.04 sec)

mysql> select * from TGrade;
+-----+---------+---------+------+
| GID | Chinese | English | ID   |
+-----+---------+---------+------+
|   5 |   91.00 |   93.00 |    3 |
+-----+---------+---------+------+
1 row in set (0.00 sec)

5）从TStudent ID中更新ID为2的数据为12，发现TGrade表中ID也跟着变化了。
mysql> update TStudent set ID=12 where name='wangwu';
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from TGrade;
+-----+---------+---------+------+
| GID | Chinese | English | ID |
+-----+---------+---------+------+
| 5 | 91.00 | 93.00 | 12 |
+-----+---------+---------+------+
1 row in set (0.00 sec)