mysql02:连表

Posted 2021-02-07 lisa-blog

1，外键补充，一提到外键，肯定最少存在两张表的关系，外键有几个写几个就好

2，创建表的时候，最后一行不要加逗号，不然会报错的

mysql> create table userinfo(
    -> id int not null auto_increment primary key,
    -> name varchar(10),
    -> department_id int,
    -> ) engine=innodb default charset=utf8;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘) engine=innodb default charset=utf8‘ at line 5

3，两张表，创建简单的外键关系，这个报错的原因是因为department 表里面的id 不是唯一索引的，这个在后面会讲的我记忆当中

mysql> create table department(
    -> id int not null,
    -> department char(20)
    -> ) engine = innodb default charset utf8
    -> ;
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> create table userinfo2(
    -> id int not null auto_increment primary key,
    -> name varchar(10),
    -> department_id int,
    -> constraint fk_1 foreign key (department_id) references department(id)
    -> ) engine=innodb default charset=utf8;
ERROR 1822 (HY000): Failed to add the foreign key constraint. Missing index for constraint ‘fk_1‘ in the referenced table ‘department‘

4，所以应用上一小节学习的知识，修改department表里面的ID属性，正确写法如下，注意modify后面没有colume:

mysql> alter table department modify id int not null auto_increment primary key; 
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

5，修改完成之后，再次建立外键关系，成功！

mysql> create table userinfo2( id int not null auto_increment primary key, name varchar(10), department_id int, constraint fk_1 foreign key (department_id) references department(id) ) engine=innodb default charset=utf8;
Query OK, 0 rows affected, 1 warning (0.01 sec)

6，做一下老师上节课的作业，创建出学生，老师，课程，成绩表，并建立约束关系