SQL 常用语句

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
 
问题：
1、查询“001”课程比“002”课程成绩高的所有学生的学号；
  select a.S# from (select s#,score from SC where C#='001') a,(select s#,score
  from SC where C#='002') b
  where a.score>b.score and a.s#=b.s#;
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.Sname,count(SC.C#),sum(score)
  from Student left Outer join SC on Student.S#=SC.S#
  group by Student.S#,Sname
4、查询姓“李”的老师的个数；
  select count(distinct(Tname))
  from Teacher
  where Tname like '李%';
5、查询没学过“叶平”老师课的同学的学号、姓名；
    select Student.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='叶平');
6、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；
  select Student.S#,Student.Sname from
Student,SC where Student.S#=SC.S# and SC.C#='001'and exists( Select * from SC as SC_2 where
SC_2.S#=SC.S# and
SC_2.C#='002');
7、查询学过“叶平”老师所教的所有课的同学的学号、姓名；
  select S#,Sname
  from Student
  where S# in (select S# from SC ,Course ,Teacher where SC.C#=Course.C# and Teacher.T#=Course.T#
and Teacher.Tname='叶平' group by S# having count(SC.C#)=(select count(C#) from Course,Teacher  where Teacher.T#=Course.T# and Tname='叶平'));
8、查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；
  Select S#,Sname from (select Student.S