（1）查询学生的基本信息；
select * from S;
（2）查询“CS”系学生的基本信息；
select * from S where Sdept =’CS’;
（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 S
where Sdept = ‘CS’ and Sage not
Between 19 and 21;
（4）找出最大年龄；
select max(Sage) from S;
（5）找出“CS”系年龄最大的学生，显示其学号、姓名；
select * from S where
Sage = (select max(Sage) from S where Sdept = "CS"); 
（6）找出各系年龄最大的学生，显示其学号、姓名；
select Sno, Sname from S
where Sage in
(select max(Sage) from S group by Sdept);
（7）统计“CS”系学生的人数；
select count(*) from S;
（8）统计各系学生的人数，结果按升序排列；
select Sdept, count(*) from S group by Sdept
Order by count(*) asc;
（9）按系统计各系学生的平均年龄，结果按降序排列；
select Sdept, avg(Sage) as AVG from
group by Sdept
order by AVG desc;
（10）查询每门课程的课程名；
select Cname from C;
（11）查询无先修课的课程的课程名和学时数；
select Cname, Ccredit from C where Cpno is null;
（12）统计无先修课的课程的学时总数；
select sum(Ccredit) from C
where Cpno is null;
（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), avg(Grade),Sum(C.Credit)
from SC,C
where SC.Cno = C.Cno
group by Sno;
（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) as num, avg(Grade) as Avg
from SC
group by Cno;
（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 S.Sdept ,avg(Grade) as Avg , S.Sname as "name" from S
left join SC on S.Sno = SC.Sno
group by Sdept,S.Sname
having avg(Grade) > 85 order by avg(Grade);
（16）查询选修了“1”或“2”号课程的学生学号和姓名；
select S.Sno, S.Sname from S, SC
where (SC.Cno = 1 or SC.Cno = 2)
and S,Sno = SC.Sno group by S.Sno, S.Sname
order by S.Sno, S.Sname;
（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 S.Sno , S.Sname ,SC.Grade from S ,SC, C
where C.Cname = "数据库"
and Grade < 60
and SC.Cno = C.Cno
and SC.Sno = S.Sno;
（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）；
select * from S,C,SC
where S.Sno = SC.Sno
and C.Cno = SC.Cno;
（20）查询没有选修课程的学生的基本信息；
select * from S
where Sno not in (select Sno from SC);
（21）查询选修了3门以上课程的学生学号；
select Sno
from SC
group by Sno
having count(Cno)>=3;
（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 S.Sno
from S left join SC on SC.Sno = S.Sno
where Grade > 80 and Grade is not null
group by S.Sno
order by S.Sno
（23）查询选修课程成绩均在80分以上的学生学号；
select Sno, Sname from S
where Sno in
(select Sno from SC group by Sno having avg(Grade) >=80);
（24）查询选修课程平均成绩在80分以上的学生学号；
select Sno, Sname from S
where Sno in
(select Sno from SC group by Sno having avg(Grade) >=80);
