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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;--查询所有用户的订单详情
select *from user left join orders on user.id=orders.user_id;--查询所有订单的用户详情
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的所有用户信息。
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;