SQL – foreign key

SQL – foreign key

Table of Contents

• 父表和子表的行为
• 父表和子表同步的例子

优点:
保持数据一致性, 完整性
实现一对一或者一对多的关系

对两个表的要求:
必须使用相同的存储引擎 – InnoDB
数字长度, 是否是 unsigned 的属性相同
字符长度可以不同
必须有索引, 没有的话自动创建

foreign key 就是定义了当父表对应的字段发生变化的时候子表的行为

父表和子表的行为

drop table if exists test1;
create table test1 (
id int auto_increment primary key,
name varchar(20) NOT NULL
);

drop table if exists test2;
create table test2 (       # error 150, 错误 类型不同
id2 smallint primary key auto_increment,
name2 varchar(10) not null,
foreign key (id2) references test1 (id)
);

drop table if exists test2;
create table test2 (
id2 int auto_increment primary key,
name2 varchar(10) not null,
foreign key (id2) references test1 (id)
);

父表: test1
子表: test2

foreign key 默认: 父表和子表的行为

on update / on delete -—

create table test3 (
id3 int primary key auto_increment,
name3 varchar(10) not null,
foreign key (id3) references test1 (id) on delete cascade on update cascade
);

父表和子表同步的例子

准备

drop table if exists test1;
drop table if exists test2;

create table test1 (
id int NOT NULL primary key,
sex enum(‘f‘, ‘m‘)
);

create table test2 (
id int not null,
name varchar(10),
foreign key (id) references test1(id) on delete cascade on update cascade
);

insert into test1 (id, sex) values (1, ‘f‘), (2, ‘m‘), (3, ‘f‘);

select * from test1;

| id | sex  |
+----+------+
|  1 | f    |
|  2 | m    |
|  3 | f    |

select * from test2;

Empty set (0.00 sec)

insert into test2 (id, name) values
(1, ‘abc‘), (1, ‘def‘), (1, ‘xyz‘),
(2, ‘ghi‘), (2, ‘jkl‘), (2, ‘opq‘),
(3, ‘lmn‘), (3, ‘uvw‘), (3, ‘rst‘);

select * from test2;

| id | name |
+----+------+
|  1 | abc  |
|  1 | def  |
|  1 | xyz  |
|  2 | ghi  |
|  2 | jkl  |
|  2 | opq  |
|  3 | lmn  |
|  3 | uvw  |
|  3 | rst  |

delete from test1 where id=1;

select * from test2;

| id | name |
+----+------+
|  2 | ghi  |
|  2 | jkl  |
|  2 | opq  |
|  3 | lmn  |
|  3 | uvw  |
|  3 | rst  |

drop table test1, test2;