MySQL通過外鍵約束實現數據庫的參照完整性，外鍵約束條件可在創建外鍵時指定，table的存儲引擎只能是InnoDB，因為只有這種存儲模式才支持外鍵。
外鍵約束條件有以下4種：
（1）restrict方式：同no action，都是立即檢查外鍵約束；
- - 限制，指的是如果子表引用父表的某個字段的值，那么不允許直接刪除父表的該值。
（2）cascade方式：在父表上update/delete記錄時，同步update/delete子表的匹配記錄 ；
On delete cascade從mysql3.23.50開始可用，on update cascade從mysql4.0.8開始可用 ；
-- 級聯，刪除/更新父表的某條記錄，子表中引用該值的記錄會自動被刪除/更新。
（3）No action方式：如果子表中有匹配的記錄,則不允許對父表對應候選鍵進行update/delete操作 這個是ANSI SQL-92標准,從mysql4.0.8開始支持；
--無參照完整性關系，有了也不生效。
（4）set null方式：在父表上update/delete記錄時，將子表上匹配記錄的列設為null 要注意子表的外鍵列不能為not null
On delete set null從mysql3.23.50開始可用；，on update set null從mysql4.0.8開始可用 。
首先創建一個用戶表，並插入兩條記錄：
mysql> create table t_group -> (id int auto_increment primary key, -> name varchar(20)) -> engine=InnoDB; Query OK, 0 rows affected (0.08 sec) mysql> insert into t_group values (1,'Group_1'),(2,'Group_2'); Query OK, 2 rows affected (0.04 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> select * from t_group; +----+---------+ | id | name | +----+---------+ | 1 | Group_1 | | 2 | Group_2 | +----+---------+ 2 rows in set (0.00 sec)
1.級聯方式
創建級聯子表：
mysql> create table t_user -> (id int not null primary key, -> name varchar(10),groupid int, -> foreign key(groupid) references t_group(id) -> on delete cascade on update cascade) -> engine=InnoDB; Query OK, 0 rows affected (0.08 sec)
完整性約束測試，以下例子都省略完整性測試：
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); //實體完整性測試，不能插入重復主鍵值
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into t_user values (3,'maiqi',3); //參照完整性測試，不能插入在主表中不存在的外鍵值
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)
級聯測試：
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; //從主表中更新一個記錄 Query OK, 1 row affected (0.02 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t_user; /改變主表數據，從表數據隨着更新 +----+--------+---------+ | 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; //從主表中刪除一個記錄 Query OK, 1 row affected (0.03 sec) mysql> select * from t_user; //子表中對應的記錄自動被刪除 +----+------+---------+ | id | name | groupid | +----+------+---------+ | 1 | dayu | 1 | +----+------+---------+ 1 row in set (0.00 sec)
2.置空模式（set null，主表記錄被刪除，從表中對應的值設置為NULL）
首先創建一個子表：
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; //從主表中刪除一個記錄
Query OK, 1 row affected (0.01 sec)
mysql> select * from t_user_1; //子表中對應的屬性值被自動設置為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，則不允許在主表中進行更新或刪除
首先創建一個子表：
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=3 where id=1; ERROR 1451 (23000): 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=2; ERROR 1451 (23000): 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.主從表的刪除
刪除從表沒有限制，如下：
mysql> drop table t_user_2; Query OK, 0 rows affected (0.01 sec)
如果存在引用的從表，則主要不能隨意刪除：
mysql> drop table t_group; ERROR 1217 (23000): Cannot delete or update a parent row: a foreign key constraint fails
經過以上測試，了解到MySQL通過外鍵約束實現了數據的完整性與一致性。