sql中group by 和count 以及内连接查询的运用

引用

问题描述:
现有3张表:分别是
学生信息表S(sno,sname)
课程信息表C(cno,cname,cteacher)
学生与课程关系表SC(sno,cno,scroce)
问题1：查询出所有未选修过李明老师所教课程的学生姓名.
问题2：查询出有2门以上未及格的学生姓名
问题3：查询出既选修课程"1"也选修了课程"2"的学生姓名

主要考察:group by 和inner join ,having条件筛选,以及count函数的使用
group by 主要是对结果集进行分组,然后采用having搭配使用进行数据的筛选,

drop table S,SC,C
create table S(
	sno varchar(15),			--学号
	sname varchar(15)			--学生姓名
)
create table C(
	cno varchar(15),			--课程编号
	cname varchar(15),			--课程名称
	cteacher varchar(15)		--授课教师
)
create table SC(
	sno varchar(15),			--学号
	cno varchar(15),			--课程编号
	scroce float				--分数
)

select * from S;
delete from S;
insert into S(sno,sname)values('s001','jerry');
insert into S(sno,sname)values('s002','tom');
insert into S(sno,sname)values('s003','jason');

delete from C;
select * from C;
insert into C(cno,cname,cteacher)values('c001','1','李明');
insert into C(cno,cname,cteacher)values('c002','2','李明0');
insert into C(cno,cname,cteacher)values('c003','3','李明1');
insert into C(cno,cname,cteacher)values('c004','4','李明2');

delete from SC;
select * from SC;
insert into SC(sno,cno,scroce)values('s001','c001',40);
insert into SC(sno,cno,scroce)values('s001','c002',50);
insert into SC(sno,cno,scroce)values('s002','c001',50);
insert into SC(sno,cno,scroce)values('s002','c003',50);

select 
	a.sno,a.sname,b.cno
from S as a
inner join SC as b on b.sno=a.sno
where not exists(
	select * from SC where sno=b.sno and cno in(select cno from C where cteacher = '李明')
);


--有2门未及格的学生
select
	a.sname
from S as a
inner join SC as b on b.sno=a.sno
where b.scroce<60
group by a.sname
having count(sname)>1;

--既选修了课程1也选修了课程2的学生姓名

select 
	a.sname
from S a 
inner join SC b on a.sno=b.sno 
inner join C c on c.cno=b.cno 
where b.cno in ('c001','c002')
group by a.sname
having count(a.sname)=2;

评论

1 楼 wing123 2011-08-17  

--有2门未及格的学生  
select 
    a.sname  
from S as a  
inner join SC as b on b.sno=a.sno  
where b.scroce<60  
group by a.sname  
having count(sname)>1;

这个有问题：不能按“a.sname”分组，如果有2个同样叫‘jerry’的人，都有两门未及格，结果就只能查出一个来