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: 0 mysql> INSERT INTO departments SELECT NULL, ‘test‘; Query OK, 1 row affected (0.01 sec) Records: 1 Duplicates: 0 Warnings: 0 mysql> 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: 0 mysql> 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: 0 mysql> 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。
时间: 2024-10-25 20:04:59