表结构：
student(s#,sname,sage,ssex)学生表
course（c#,cname,T#）课程表
sc(s#,c#,score)成绩表
Teacher(T#,tname)教师表
1.查询001课程比002课程成绩高的所有学生的学号：
select sc1.s#
from sc sc1 join sc sc2 on sc1.s# = sc2.s#
where sc1.c# = ‘001’ and sc2.c# = ‘002’ and sc1.score > sc2.score
2.查询平均成绩大于60分的同学的学号和平均成绩：
select s#, avg(score) from sc group by s# having avg(score)>60;
3.查询所有同学的学号，姓名，选课数，总成绩：
select student.s#,student.name ,count(sc.c#) ,sum(sc.score)
from student left join sc on student.s# = sc.s# 注：内连接要计算笛卡尔积，这里使用左外连接效率更高
4.查询姓李的老师个数：
select count(T#) from Teacher where tname like “李%”
注：模糊查询, %代表任意字符，_代表任一单个字符，[……]代表是括号中字符中的单个字符, [^……]代表不是括号中字符中的单个字符
5.查询没有学过叶萍老师课的同学学号，姓名：
select studet.s# student.sname
from student
where s# not in (select distinct(sc.s#) from sc, course, teacher where sc.c# = course.c# and teacher.t# = course.t# and teacher.tname = “叶萍”)；
三表联合 + 子查询
6.查询学过001和002课程的同学的姓名学号：
select student.s3 student .sname
from student join course on student.s# = course.s#
where(course.c# = ‘001’ union course.c# = ‘002’);
7.查询学过叶萍老师课的同学的学号，姓名：
select student.s# student.sname
from student
where s# in(select distinct(sc.s#) from sc course teacher where sc.c# = course.c# and teacher.t# = course.t# and teacher.tnmae = “叶萍”) 注：distinct为去重函数
8.查询002成绩比001低的同学的学号，姓名：
select student.s# student .name
from student where student.s# in(select sc1.s# from sc sc1 join sc sc2 on sc1.s# = sc2.s# where s1.c# = 001 and s2.c# = 002 and sc1.score> sc2.score);
9.查询所有课程成绩小于60分的同学的学号，姓名：
select student.s# student.sname from student join sc on student.s# = sc.s#
where sc.score > 60;
10.查询没有学全 所有课的同学的学号和姓名：
select student.s# student.sname from student,sc
where student.s# = sc.s# group by student.s#,student.sname having count(c#) < (select count(c#)from course);
11.查询至少有一门课程与学号1001的同学所学相同的同学的学号和姓名：
select student.s# ,student.sname from student join sc
where student.s# = sc.s# and c# in select c# from sc where s# = ‘1001’
12.查询至少学过学号为001同学所有一门课的其他同学的学号和姓名：
select distinct student.s#,student,sname from
SC join student on SC.s# = student.s#
where c# in(select c# from sc where s#=’001’);