增加外键

增加外键约束时，假定A参照B，那么B中被参照的列必须是主键，否则语句无法执行

mysql> desc user_info;

ERROR 1146 (42S02): Table 'projectdevlop.user_info' doesn't exist
mysql> show tables;
+-------------------------+
| Tables_in_projectdevlop |
+-------------------------+
| menu_info               |
| role_info               |
| role_menu_info          |
| user_role_info          |
+-------------------------+
4 rows in set (0.00 sec)

mysql> CREATE TABLE USER_INFO(
    ->     id int(8) not null auto_increment,
    ->     userid varchar(8) primary key  NOT NULL,
    ->     username varchar(32),
    ->     userpassword varchar(32),
    ->     userstatus varchar(2)
    -> );
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a ke
mysql> CREATE TABLE USER_INFO(
    ->     id int(8) not null ,
    ->     userid varchar(8) primary key  NOT NULL,
    ->     username varchar(32),
    ->     userpassword varchar(32),
    ->     userstatus varchar(2)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> desc user_info;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| id           | int(8)      | NO   |     | NULL    |       |
| userid       | varchar(8)  | NO   | PRI | NULL    |       |
| username     | varchar(32) | YES  |     | NULL    |       |
| userpassword | varchar(32) | YES  |     | NULL    |       |
| userstatus   | varchar(2)  | YES  |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

mysql> alter table user_role_INFO add foreign key user_role_INFO_userid (userid) references USER_INFO(userid);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0