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

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)
);

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

3.添加外键列
alter table product add category_id varchar(32);

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

5.订单表
create table orders(
oid varchar(32) primary key,
totalprice double
);

6.订单项表
create table orderitem(
oid varchar(50),
pid varchar(50)
);

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

8.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);

9.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','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');

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

2.从orderitem到orders的关系