MYSQL 数据库给表加外键约束条件数据库给表加外键约束条件 (史上最详细教程史上最详细教程)
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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