1. 背景

* MySQL有两种常用的引擎类型MyISAM和InnoDB。目前只有InnoDB引擎类型支持外键约束。

* 本表的列必须与外键类型相同, 外键必须是外表的唯一键(或主键)。

* 设置外建的列不能设置 NO NULL 字段属性。

2. 外建作用

* 使两张表形成关联,外键只能引用外表中的列的值

* 保持数据一致性，完整性，控制存储在外键表中的数据

3. 外键实验 [ 员工 --> 部门 ]

* 创建外键依赖的外表 departments

mysql> CREATE TABLE departments(-> id BIGINT PRIMARY KEY NOT NULL AUTO_INCREMENT,-> name VARCHAR(64) NOT NULL-> )ENGINE=INNODB CHARSET=utf8mb4;
Query OK, 0 rows affected (0.05 sec)

* 创建员工表 empoyees, 并建立外键

指定外键关键字： FOREIGN KEY(列名)

引用外键关键字： REFERENCES <外键表名>(外键列名)　

事件触发限制:  [默认] no action

ON DELETE  SET NULL   当外表字段删除时，本表设置为NULL(空值) [ 不推荐设置此荐 ]

ON UPDATE CASCADE   当外表字段更新时，本表外键级联更新

ON DELETE/UPDATE SET DEFAULT 当有事件触发时，设置为默认值

　　　　ON DELETE/UPDATE RESTRICT      当有事件触发时，限制外表中的外键改动

mysql> CREATE TABLE empoyees( -> id BIGINT PRIMARY KEY NOT NULL AUTO_INCREMENT,-> sex ENUM('M', 'F') NOT NULL,-> age INT NOT NULL,-> department BIGINT,-> FOREIGN KEY(department) -> REFERENCES departments(id) -> ON DELETE SET NULL-> ON UPDATE CASCADE-> )ENGINE=INNODB CHARSET=utf8mb4;
Query OK, 0 rows affected (0.12 sec)

* 对部门表 departments 插入数据

mysql> INSERT INTO departments SELECT NULL, 'dev';
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0mysql> INSERT INTO departments SELECT NULL, 'test';
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0mysql> INSERT INTO departments SELECT NULL, 'ops';
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0

* 查看部门表数据

mysql> SELECT * FROM departments;
+----+------+
| id | name |
+----+------+
|  1 | dev  |
|  2 | test |
|  3 | ops  |
+----+------+
3 rows in set (0.00 sec)

* 插入正常数据 （部门编号存在于部门表中）

mysql> INSERT INTO empoyees SELECT NULL, 'M', 22, 2;
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0mysql> SELECT * FROM empyees;
ERROR 1146 (42S02): Table 'mytest.empyees' doesn't exist
mysql> SELECT * FROM empoyees;
+----+-----+-----+------------+
| id | sex | age | department |
+----+-----+-----+------------+
|  1 | M   |  22 |          2 |
+----+-----+-----+------------+
1 row in set (0.01 sec)

* 插入非正常数据 (部门编号不存在于部门表中)　[ 部门表中不存在id为4的列 ]

mysql> INSERT INTO empoyees SELECT NULL, 'M', 22, 4;
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`mytest`.`empoyees`, CONSTRAINT `empoyees_ibfk_1` FOREIGN KEY (`department`) REFERENCES `departments` (`id`) ON DELETE SET NULL ON UPDATE CASCADE)

* 修改部门表数据 [ 员工表中已有数据关联部门表中id为2 ]

mysql> SELECT * FROM empoyees;
+----+-----+-----+------------+
| id | sex | age | department |
+----+-----+-----+------------+
|  1 | M   |  22 |          2 |
+----+-----+-----+------------+
1 row in set (0.01 sec)mysql> UPDATE departments SET id = 4 WHERE id=2;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 0mysql> SELECT * FROM empoyees;
+----+-----+-----+------------+
| id | sex | age | department |
+----+-----+-----+------------+
|  1 | M   |  22 |          4 |
+----+-----+-----+------------+
1 row in set (0.01 sec)

* 删除部门表数据 [ 员工表中已有数据关联部门表中id为4 ]

mysql> SELECT * FROM empoyees;
+----+-----+-----+------------+
| id | sex | age | department |
+----+-----+-----+------------+
|  1 | M   |  22 |          2 |
+----+-----+-----+------------+
1 row in set (0.01 sec)mysql> DELETE FROM departments WHERE id = 4;
Query OK, 1 row affected (0.01 sec)mysql> SELECT * FROM empoyees;
+----+-----+-----+------------+
| id | sex | age | department |
+----+-----+-----+------------+
|  1 | M   |  22 |       NULL |
+----+-----+-----+------------+
1 row in set (0.00 sec)

4. 总结

以需求驱动技术，技术本身没有优略之分，只有业务之分。