----外键约束；
----涉及到两个表：父表，子表；
----主表和副表。

--班级
create table classes(
    id int primary key,
    name varchar(20)
    );


--学生表
create table students(
    id int primary key,
    name varchar(20),
    class_id int,
    foreign key(class_id) references classes(id) 
    );

mysql> desc classes;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   | PRI | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.03 sec)

mysql> desc students;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int         | NO   | PRI | NULL    |       |
| name     | varchar(20) | YES  |     | NULL    |       |
| class_id | int         | YES  | MUL | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> 
mysql> mysql> insert into classes 1,"yiban");
Query OK, 1 row affected (0.01 sec)

mysql> insert into classes values(2,"erban");
Query OK, 1 row affected (0.00 sec)

mysql> insert into classes values(3,"sanban");
Query OK, 1 row affected (0.00 sec)

mysql> select * from classes;
+----+--------+
| id | name   |
+----+--------+
|  1 | yiban  |
|  2 | erban  |
|  3 | sanban |
+----+--------+
3 rows in set (0.00 sec)

mysql> insert into students values(1001,"zhangsan",1);
Query OK, 1 row affected (0.00 sec)

mysql> insert into students values(1002,"lisi",2);
Query OK, 1 row affected (0.00 sec)

mysql> insert into students values(1003,"lisi",3);
Query OK, 1 row affected (0.01 sec)

mysql> insert into students values(1004,"wangmazi",4);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`students`, CONSTRAINT `students_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))
副表是需要参照主表

mysql> select * from students;
+------+----------+----------+
| id   | name     | class_id |
+------+----------+----------+
| 1001 | zhangsan |        1 |
| 1002 | lisi     |        2 |
| 1003 | lisi     |        3 |
+------+----------+----------+
3 rows in set (0.01 sec)

mysql> 


---结论；

--1. 主表(父表)classes 中没有的数据值，在副表（子表）中，是不可以使用的；
--2. 主表中的记录被副表引用，是不可以被删除的。

mysql> delete from classes where id=3;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test`.`students`, CONSTRAINT `students_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))
mysql>