分頁操作：使用limit(參數1，參數2)
    起始位置(參數1)=(第幾頁-1)*每頁顯示的條數(參數2)
    
1.分類表
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.添加外鍵列
alter table product add category_id varchar(32);

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

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

6.訂單項表
create table orderitem(
    oid varchar(50),
    pid varchar(50)
);

7.聯合主鍵
alter table orderitem add primary key(oid,pid);

8.訂單表和訂單項表的主外鍵關系
alter table orderitem add constraint orderitem_orders_fk foreign key(oid) references orders(oid);

9.商品表和訂單項表的主外鍵關系
 
alter table orderitem add constraint orderitem_product_fk foreign key(pid) references product(pid);



insert into category(cid,cname) values(‘c001‘,‘家電‘);
insert into category(cid,cname) values(‘c002‘,‘服飾‘);
insert into category(cid,cname) values(‘c003‘,‘化妝品‘);

insert into product(pid,pname,price,category_id) values(‘p001‘,‘聯想‘,‘5000‘,‘c001‘);
insert into product(pid,pname,price,category_id) values(‘p002‘,‘海爾‘,‘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‘,‘真維斯‘,‘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‘,‘勁霸‘,‘2000‘,‘c002‘);

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



聲明外鍵約束
1.從orderitem到product的關系
alter table orderitem add foreign key(pid) references product(pid);

1.1 刪除外鍵約束
alter table 從表 drop foreign key 外鍵名稱


2.從orderitem到orders的關系

MySQL學習(二)——MySQL多表