--1．   查询学生的基本信息；
select * from student
go
--2．   查询“CS”系学生的基本信息；
select * from student
where sdept='cs'
go
--3．   查询“CS”系学生年龄不在19到21之间的学生的学号、姓名；
select sno,sname
from student
where sdept='cs' and sage not between 19 and 21
go
--4．   找出最大年龄；
select max(sage)
from student
go
--5．   找出“CS”系年龄最大的学生，显示其学号、姓名；
select sno,sname
from student
where sdept='cs' and sage=(select max(sage)
                           from student
                           where sdept='cs'
                          )
go
--6．   找出各系年龄最大的学生，显示其学号、姓名；
select sno,sname,sdept
from student s1
where sage=(select max(sage)
            from student
            where sdept=s1.sdept
           )
go
--7．   统计“CS”系学生的人数；
select count(*)
from student
where sdept='cs'
go
 
--8．   统计各系学生的人数，结果按升序排列；
select sdept,count(sno) as sm
from student
group by sdept
order by sm
go
**********************************************


select sdept,count(*) as dd

from student

group by sdept

order by dd

go

--9．   按系统计各系学生的平均年龄，结果按降序排列；
select sdept,count(sage) as sm
from student
group by sdept
order by sm desc
go
select sdept,avg（sage）dd

from student

group by sdept

order by dd desc

go


--10．  查询每门课程的课程名；
select cno,cname from course
go
select cname from course

go


--11．  查询无先修课的课程的课程名和学分；
select cname,ccredit
from course
where cpno is null
go
--12．  统计无先修课的课程的学分总数；//select * from course
select sum(ccredit)
from course
where cpno is null
go
--13．  统计每位学生选修课程的门数、学分及其平均成绩；
select sno,count(sc.cno),count(ccredit),avg(grade)
from sc,course
where sc.cno=course.cno
group by sno
go
--14．  统计选修每门课程的学生人数及各门课程的平均成绩；
select cno,count(sno),avg(grade)
from sc
group by cno
go
--15．  找出平均成绩在85分以上的学生，结果按系分组，并按平均成绩的升序排列；
select sdept,sc.sno,avg(grade)
from sc,student
where sc.sno=student.sno
group by sc.sno,sdept
having avg(grade)>85
order by avg(grade)

go
--16．  查询选修了“1”或“2”号课程的学生学号和姓名；
select  distinct sc.sno,sname
from sc,student
where sc.sno=student.sno and (sc.cno='1' or sc.cno='2')
 
go 
--17．  查询选修了“1”和“2”号课程的学生学号和姓名；
select distinct sc.sno,sname
from sc,student
where sc.sno=student.sno and sc.cno='1' and sc.sno in(select sno
                                                     from sc
                                                   where cno='2')
go

**********************************

7、将“CS”系全体学生的成绩置零； update sc set grade=0

where sno in      ( selsct sno        from student        where sdept='cs')

go



*****************************************




--18．  查询选修了课程名为“数据库系统”且成绩在60分以下的学生的学号、姓名和成绩；
select sc.sno,sname,grade
from sc,student,course
where sc.sno=student.sno and sc.cno=course.cno and course.cname='数据库系统' and grade<60
go
--19．  查询每位学生选修了课程的学生信息（显示：学号，姓名，课程号，课程名，成绩）；
selsct sc.sno,sname,sc.cno,cname,grade

from student,sc,course

where student.sno=sc.sno and course.cno=sc.cno

go



--20 查询没有选修课程的学生的基本信息；
select sno,sname,sage
from student
where sno not in(select distinct sno from sc)
go
select *

from student

where sno not in(select distinct sno from sc)

go


--21．  查询选修了3门及以上课程的学生学号；
select sno
from sc
group by sno
having count(cno)>=3
go
--22．  查询选修课程成绩至少有一门在80分以上的学生学号；
select distinct sno
from sc
where grade>80
go
--23．  查询选修课程成绩均在80分以上的学生学号；
select distinct sno
from sc
where sno not in(select distinct sno
                from sc
                where grade<80)
go 
--24．  查询选修课程平均成绩在80分以上的学生学号；
select sno
from sc
group by sno
having avg(grade)>80
go

select distinct sno

from sc

where 80<(select   min(grade)      

                from sc  y

                where y.sno=x.sno  )

go