MYSQL 数据库给表加外键约束条件 (史上最详细教程)

下面是已经创建好的两个数据表！(还未加外键约束)
1.

mysql> show columns from message;//信息表
+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| m_id      | int(11)      | NO   | PRI | NULL    | auto_increment |
| m_name    | varchar(60)  | NO   |     | NULL    |                |
| m_sex     | varchar(16)  | NO   |     | NULL    |                |
| m_brith   | date         | YES  |     | NULL    |                |
| m_address | varchar(250) | YES  |     | NULL    |                |
| m_c_id    | int(11)      | YES  |     | NULL    |                |
+-----------+--------------+------+-----+---------+----------------+

mysql> show columns from course;//课程表
+---------+--------------+------+-----+---------+----------------+
| Field   | Type         | Null | Key | Default | Extra          |
+---------+--------------+------+-----+---------+----------------+
| c_id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| c_name  | varchar(150) | NO   |     | NULL    |                |
| c_type  | varchar(60)  | NO   |     | NULL    |                |
| c_score | int(11)      | NO   |     | NULL    |                |
+---------+--------------+------+-----+---------+----------------+

下来来给message表的m_c_id字段加外键

mysql> alter table message add foreign key(m_c_id) references course(c_id);//命令
Query OK, 0 rows affected (0.84 sec)//成功！

看一下表的结构

mysql> show columns from message;//命令
+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| m_id      | int(11)      | NO   | PRI | NULL    | auto_increment |
| m_name    | varchar(60)  | NO   |     | NULL    |                |
| m_sex     | varchar(16)  | NO   |     | NULL    |                |
| m_brith   | date         | YES  |     | NULL    |                |
| m_address | varchar(250) | YES  |     | NULL    |                |
| m_c_id    | int(11)      | YES  | MUL | NULL    |                |
+-----------+--------------+------+-----+---------+----------------+

可以看到外键成功添加了
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