【398天】我爱刷题系列157（2018.03.10）

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SQL习题052——MySQL多表查询基础巩固

--新建一个day09_exercise的数据库

create database day09_exercise;
use day09_exercise;

-- 用户表(user)
create table user (

`id` int auto_increment primary key,                
      `username` varchar(50)  -- 用户姓名                                                
    );

-- 订单表(orders)
create table orders (

`id` int  auto_increment primary key,                                  
      `price` double,                                           
      `user_id` int                                       
    );

-- 给订单表添加外键约束
alter table orders add constraint user_fk foreign key (user_id) references user(id);

-- 向user表中添加数据

insert into user values(3,'张三');
    insert into user values(4,'李四');
    insert into user values(5,'王五');
    insert into user values(6,'赵六');

-- 向orders 表中插入数据

insert into orders values(1,1314,3);
    insert into orders values(2,1314,3);
    insert into orders values(3,15,4);
    insert into orders values(4,315,5);
    insert into orders values(5,1014,null);

练习:
--查询用户的订单,没有订单的用户不显示

select *from user,orders where user.id=orders.user_id;
或者select * from user join orders on user.id=orders.user_id;

--查询全部用户的订单详情

select *from user left join orders on user.id=orders.user_id;

--查询全部订单的用户详情

select *from user right join orders on user.id=orders.user_id;

练习:
--查看用户为张三的订单详情

思路:查看用户为张三的订单详情
    select id from user where username = "张三";
    select * from orders where user_id = ?
        
合二为一:
答案:select * from orders where user_id = (select id from user where username = "张三");

--查询出订单的价格大于300的全部用户信息。

思路:select user_id from orders where price>300;-- 3 3 5 null
select * from user where id in ();
            
合二为一:
答案:select * from user where id in (select user_id from orders where price>300);

--查询订单价格大于300的订单信息及相关用户的信息。

SELECT *FROM USER,orders WHERE user.id=orders.user_id AND orders.price > 300;