1. 背景

* MySQL有兩種常用的引擎類型MyISAM和InnoDB。目前只有InnoDB引擎類型支持外鍵約束。

* 本表的列必須與外鍵類型相同, 外鍵必須是外表的主鍵

* 設置外建的列不能設置 NO NULL 字段屬性。

2. 外建作用

* 使兩張表形成關聯,外鍵只能引用外表中的列的值

* 保持數據一致性，完整性，控制存儲在外鍵表中的數據

3. 外鍵實驗 [ 員工 --> 部門 ]

* 創建外鍵依賴的外表 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)

* 創建員工表 empoyees, 並建立外鍵

指定外鍵關鍵字： FOREIGN KEY(列名)

引用外鍵關鍵字： REFERENCES <外鍵表名>(外鍵列名)　

事件觸發限制: [默認] no action

ON DELETE SET NULL 當外表字段刪除時，本表設置為NULL(空值) [ 不推薦設置此薦 ]

ON UPDATE CASCADE 當外表字段更新時，本表外鍵級聯更新

ON DELETE/UPDATE SET DEFAULT 當有事件觸發時，設置為默認值

　　　　ON DELETE/UPDATE RESTRICT 當有事件觸發時，限制外表中的外鍵改動

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)

* 對部門表 departments 插入數據

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

* 查看部門表數據

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

* 插入正常數據 （部門編號存在於部門表中）

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)

* 插入非正常數據 (部門編號不存在於部門表中)　[ 部門表中不存在id為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)

* 修改部門表數據 [ 員工表中已有數據關聯部門表中id為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)

* 刪除部門表數據 [ 員工表中已有數據關聯部門表中id為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. 總結

以需求驅動技術，技術本身沒有優略之分，只有業務之分。

本文出自 “sea” 博客，請務必保留此出處http://lisea.blog.51cto.com/5491873/1943689

MySQL DDL操作--------外鍵最佳實戰