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;
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;
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;
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';