myslql主外键约束

Posted 2021-11-09 sacai

多表操作

    1.分类表

create table category(
cid varchar(32) primary key,
cname varchar(100)
);

2.商品表
create table product(
pid varchar(32) primary key,
pname varchar(40),
price double,
category_id varchar(32)
);


insert into category(cid,cname) values(‘c001‘,‘家电‘);
insert into category(cid,cname) values(‘c002‘,‘服饰‘);
insert into category(cid,cname) values(‘c003‘,‘化妆品‘);


insert into product(pid,pname,price,category_id) values(‘p001‘,‘联想‘,‘5000‘,‘c001‘);
insert into product(pid,pname,price,category_id) values(‘p002‘,‘海尔‘,‘5000‘,‘c001‘);
insert into product(pid,pname,price,category_id) values(‘p003‘,‘雷神‘,‘5000‘,‘c001‘);


insert into product(pid,pname,price,category_id) values(‘p004‘,‘JACK JONES‘,‘800‘,‘c002‘);
insert into product(pid,pname,price,category_id) values(‘p005‘,‘真维斯‘,‘200‘,‘c002‘);
insert into product(pid,pname,price,category_id) values(‘p006‘,‘花花公子‘,‘440‘,‘c002‘);
insert into product(pid,pname,price,category_id) values(‘p007‘,‘劲霸‘,‘2000‘,‘c002‘);


insert into product(pid,pname,price,category_id) values(‘p008‘,‘香奈儿‘,‘800‘,‘c003‘);
insert into product(pid,pname,price,category_id) values(‘p009‘,‘相宜本草‘,‘200‘,‘c003‘);

3.联合主外键

alter table product add foreign key(category_id) references category(cid);

4.声明外键约束

要想删除主表中与从表有关联关系的数据，可以这么做：

1. 解除主从表的约束关系
2. 先删除从表中与主表有关系的数据，再删除主表中的数据。

delete  from product where pid in (‘p008‘,‘p009‘)