MySQL 50题解析

Link： https://zhuanlan.zhihu.com/p/32137597

create table Student(SId varchar(10),Sname varchar(10),Sage datetime,Ssex varchar(10));
insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('09' , '张三' , '2017-12-20' , '女');
insert into Student values('10' , '李四' , '2017-12-25' , '女');
insert into Student values('11' , '李四' , '2017-12-30' , '女');
insert into Student values('12' , '赵六' , '2017-01-01' , '女');
insert into Student values('13' , '孙七' , '2018-01-01' , '女');


create table Course(CId varchar(10),Cname nvarchar(10),TId varchar(10));
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');


create table Teacher(TId varchar(10),Tname varchar(10));
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');


create table SC(SId varchar(10),CId varchar(10),score decimal(18,1));
insert into SC values('01' , '01' , 80);
insert into SC values('01' , '02' , 90);
insert into SC values('01' , '03' , 99);
insert into SC values('02' , '01' , 70);
insert into SC values('02' , '02' , 60);
insert into SC values('02' , '03' , 80);
insert into SC values('03' , '01' , 80);
insert into SC values('03' , '02' , 80);
insert into SC values('03' , '03' , 80);
insert into SC values('04' , '01' , 50);
insert into SC values('04' , '02' , 30);
insert into SC values('04' , '03' , 20);
insert into SC values('05' , '01' , 76);
insert into SC values('05' , '02' , 87);
insert into SC values('06' , '01' , 31);
insert into SC values('06' , '03' , 34);
insert into SC values('07' , '02' , 89);
insert into SC values('07' , '03' , 98);

1. 查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数

首先找到所有选修01课程和02课程的学生：

select sid, score from sc where sc.cid='01' as t1

select sid, score from sc where sc.cid='02' as t2

然后在从两个表里找到，sid相同（说明是同一个学生），并且t1的分比t2高的

select * from t1,t2 where t1.sid=t2.sid and t1.score>t2.score



select * 
from (select sid, score from sc where sc.cid='01') as t1,
(select sid, score from sc where sc.cid='02') as t2 
where t1.sid=t2.sid 
and t1.score>t2.score;

View Code

1.1 查询同时存在" 01 "课程和" 02 "课程的学生情况

select * from 选修01的学生 as t1，修02的学生 as t2 where t1.sid==t2.sid
select * 
from (select sid from sc where sc.cid='01') as t1,
(select sid from sc where sc.cid='02') as t2
where t1.sid=t2.sid;

View Code

1.2 查询存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null)

这个显然就是left join 的情况了

select * 
from (select sid from sc where sc.cid='01') as t1
left join (select sid from sc where sc.cid='02') as t2
on t1.sid=t2.sid;

View Code

1.3 查询不存在" 01 "课程但存在" 02 "课程的情况

如果直接用right join的特殊情况，会出现null

select *
from sc
where sc.sid not in (select sid from sc where sc.cid='01')
and sc.cid='02';

View Code