MySQL通过外键约束实现数据库的参照完整性，外键约束条件可在创建外键时指定，table的存储引擎只能是InnoDB，因为只有这种存储模式才支持外键。
外键约束条件有以下4种：
（1）restrict方式：同no action，都是立即检查外键约束；
- - 限制，指的是如果子表引用父表的某个字段的值，那么不允许直接删除父表的该值。
（2）cascade方式：在父表上update/delete记录时，同步update/delete子表的匹配记录 ；
On delete cascade从mysql3.23.50开始可用，on update cascade从mysql4.0.8开始可用 ；
-- 级联，删除/更新父表的某条记录，子表中引用该值的记录会自动被删除/更新。
（3）No action方式：如果子表中有匹配的记录,则不允许对父表对应候选键进行update/delete操作 这个是ANSI SQL-92标准,从mysql4.0.8开始支持；
--无参照完整性关系，有了也不生效。
（4）set null方式：在父表上update/delete记录时，将子表上匹配记录的列设为null 要注意子表的外键列不能为not null
On delete set null从mysql3.23.50开始可用；，on update set null从mysql4.0.8开始可用 。
首先创建一个用户表，并插入两条记录：
mysql> create table t_group
-> (id int auto_increment primary key,
-> name varchar())
-> engine=InnoDB;
Query OK, rows affected (0.08 sec) mysql> insert into t_group values (,'Group_1'),(,'Group_2');
Query OK, rows affected (0.04 sec)
Records: Duplicates: Warnings: mysql> select * from t_group;
+----+---------+
| id | name |
+----+---------+
| | Group_1 |
| | Group_2 |
+----+---------+
rows in set (0.00 sec)
1.级联方式
创建级联子表：
mysql> create table t_user
-> (id int not null primary key,
-> name varchar(),groupid int,
-> foreign key(groupid) references t_group(id)
-> on delete cascade on update cascade)
-> engine=InnoDB;
Query OK, rows affected (0.08 sec)
完整性约束测试，以下例子都省略完整性测试：
mysql> insert into t_user values (1,'dayu',1),(2,'duoduo',2);
Query OK, 2 rows affected (0.08 sec)
Records: 2 Duplicates: 0 Warnings: 0 mysql> insert into t_user values (1,'huanhuan',1); //实体完整性测试，不能插入重复主键值
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into t_user values (3,'maiqi',3); //参照完整性测试，不能插入在主表中不存在的外键值
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`t_user`, CONSTRAINT `t_user_ibfk_1` FOREIGN KEY (`groupid`) REFERENCES `t_group` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)
级联测试：
mysql> select * from t_user;
+----+--------+---------+
| id | name | groupid |
+----+--------+---------+
| 1 | dayu | 1 |
| 2 | duoduo | 2 |
+----+--------+---------+
2 rows in set (0.00 sec) mysql> update t_group set id=3 where id=2; //从主表中更新一个记录
Query OK, 1 row affected (0.02 sec)
Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t_user; /改变主表数据，从表数据随着更新
+----+--------+---------+
| id | name | groupid |
+----+--------+---------+
| 1 | dayu | 1 |
| 2 | duoduo | 3 |
+----+--------+---------+
2 rows in set (0.00 sec)
mysql> select * from t_user;
+----+--------+---------+
| id | name | groupid |
+----+--------+---------+
| 1 | dayu | 1 |
| 2 | duoduo | 3 |
+----+--------+---------+
2 rows in set (0.00 sec) mysql> delete from t_group where id=3; //从主表中删除一个记录
Query OK, 1 row affected (0.03 sec) mysql> select * from t_user; //子表中对应的记录自动被删除
+----+------+---------+
| id | name | groupid |
+----+------+---------+
| 1 | dayu | 1 |
+----+------+---------+
1 row in set (0.00 sec)
2.置空模式（set null，主表记录被删除，从表中对应的值设置为NULL）
首先创建一个子表：
mysql> select * from t_group;
+----+---------+
| id | name |
+----+---------+
| 1 | Group_1 |
| 2 | Group_2 |
+----+---------+
2 rows in set (0.00 sec)
mysql> create table t_user_1
-> (id int not null primary key,
-> name char(10),groupid int,
-> foreign key(groupid) references t_group(id)
-> on delete set null on update set null)
-> engine=InnoDB;
Query OK, 0 rows affected (0.11 sec) mysql> insert into t_user_1 values (1,'dayu',1),(2,'duoduo',2),(3,'huanhuan',2),(4,'maiqi',1);
Query OK, 4 rows affected (0.02 sec)
Records: 4 Duplicates: 0 Warnings: 0 mysql> select * from t_user_1;
+----+----------+---------+
| id | name | groupid |
+----+----------+---------+
| 1 | dayu | 1 |
| 2 | duoduo | 2 |
| 3 | huanhuan | 2 |
| 4 | maiqi | 1 |
+----+----------+---------+
4 rows in set (0.00 sec)
mysql> update t_group set id=3 where id=1;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t_user_1;
+----+----------+---------+
| id | name | groupid |
+----+----------+---------+
| 1 | dayu | NULL |
| 2 | duoduo | 2 |
| 3 | huanhuan | 2 |
| 4 | maiqi | NULL |
+----+----------+---------+
4 rows in set (0.00 sec) mysql> delete from t_group where id=2; //从主表中删除一个记录
Query OK, 1 row affected (0.01 sec) mysql> select * from t_user_1; //子表中对应的属性值被自动设置为NULL
+----+----------+---------+
| id | name | groupid |
+----+----------+---------+
| 1 | dayu | NULL |
| 2 | duoduo | NULL |
| 3 | huanhuan | NULL |
| 4 | maiqi | NULL |
+----+----------+---------+
4 rows in set (0.00 sec)
3.禁止模式（no action/restrict），如果在子表中有引用，则不允许在主表中进行更新或删除
首先创建一个子表：
mysql> select * from t_group;
+----+---------+
| id | name |
+----+---------+
| 1 | Group_1 |
| 2 | Group_2 |
+----+---------+
2 rows in set (0.00 sec) mysql> create table t_user_2
-> (id int not null primary key,
-> name char(10),groupid int,
-> foreign key(groupid) references t_group(id)
-> on delete no action on update restrict)
-> engine=InnoDB;
Query OK, 0 rows affected (0.11 sec) mysql> insert into t_user_2 values (1,'dayu',1),(2,'duoduo',2),(3,'huanhuan',2),(4,'maiqi',1);
Query OK, 4 rows affected (0.02 sec)
Records: 4 Duplicates: 0 Warnings: 0 mysql> select * from t_user_2;
+----+----------+---------+
| id | name | groupid |
+----+----------+---------+
| 1 | dayu | 1 |
| 2 | duoduo | 2 |
| 3 | huanhuan | 2 |
| 4 | maiqi | 1 |
+----+----------+---------+
4 rows in set (0.00 sec)
mysql> update t_group set id= where id=;
ERROR (): Cannot delete or update a parent row: a foreign key constraint fails (`test`.`t_user_2`, CONSTRAINT `t_user_2_ibfk_1` FOREIGN KEY (`groupid`) REFERENCES `t_group` (`id`) ON DELETE NO ACTION)
mysql> delete from t_group where id=;
ERROR (): Cannot delete or update a parent row: a foreign key constraint fails (`test`.`t_user_2`, CONSTRAINT `t_user_2_ibfk_1` FOREIGN KEY (`groupid`) REFERENCES `t_group` (`id`) ON DELETE NO ACTION)
4.主从表的删除
删除从表没有限制，如下：
mysql> drop table t_user_2;
Query OK, rows affected (0.01 sec)
如果存在引用的从表，则主要不能随意删除：
mysql> drop table t_group;
ERROR (): Cannot delete or update a parent row: a foreign key constraint fails
经过以上测试，了解到MySQL通过外键约束实现了数据的完整性与一致性。