MySQL学习(二)——MySQL多表

分页操作：使用limit(参数1，参数2)
    起始位置(参数1)=(第几页-)*每页显示的条数(参数2)

.分类表
create table category(
    cid varchar() primary key,
    cname varchar()
);

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

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

.添加外键列
alter table product add category_id varchar();

.添加约束
alter table product add constraint product_fk foreign key(category_id) references category(cid);

.订单表
create table orders(
    oid varchar() primary key,
    totalprice double
);

.订单项表
create table orderitem(
    oid varchar(),
    pid varchar()
);

.联合主键
alter table orderitem add primary key(oid,pid);

.订单表和订单项表的主外键关系
alter table orderitem add constraint orderitem_orders_fk foreign key(oid) references orders(oid);

.商品表和订单项表的主外键关系
alter table orderitem add constraint orderitem_product_fk foreign key(pid) references product(pid);

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','','c001');
insert into product(pid,pname,price,category_id) values('p002','海尔','','c001');
insert into product(pid,pname,price,category_id) values('p003','雷神','','c001');

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

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

声明外键约束
.从orderitem到product的关系
alter table orderitem add foreign key(pid) references product(pid);

1.1 删除外键约束
alter table 从表 drop foreign key 外键名称

.从orderitem到orders的关系