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