題目：http://www.cnblogs.com/wupeiqi/articles/5729934.html
2、查詢“生物”課程比“物理”課程成績高的所有學生的學號；
3、查詢平均成績大於60分的同學的學號和平均成績；
select a.sid,avg(number) from student a,score b where a.sid=b.student_id group by a.sid having avg(number) > 60;
4、查詢所有同學的學號、姓名、選課數、總成績；
select a.sid,sname,count(b.sid) as course_num,sum(b.number) as scores from student a left join score b on a.sid=b.student_id group by a.sid;
5、查詢姓“李”的老師的個數；
select count(tid) from teacher where tname like '李%';
6、查詢沒學過“葉平”老師課的同學的學號、姓名；
select student.sid,sname from student where student.sid not in (select student_id from score where course_id in (select cid from course,teacher where teacher_id=tid and tname='葉平'));
查詢學過“葉平”老師課的同學的學號、姓名；
select student_id,sname from score,student where student.sid=student_id and course_id in (select cid from course,teacher where teacher_id=tid and tname='葉平') group by student_id;
7、查詢學過“001”並且也學過編號“002”課程的同學的學號、姓名；
select s1.sid from student s1 where (select count(*) from score s2 where s2.student_id=s1.sid and s2.course_id=1)>0 and (select count(*) from score s3 where s3.student_id=s1.sid and s3.course_id=2)>0;
8、查詢學過“葉平”老師所教的所有課的同學的學號、姓名；（若在表score上建立 student_id 和 course_id 的唯一索引）
select student_id,sname from score,student where student_id=student.sidnd course_id in (select cid from course,teacher where teacher_id=tid and tname=波多') group by student_id having count(*)>=2;
9、查詢課程編號“002”的成績比課程編號“001”課程低的所有同學的學號、姓名；
select a.student_id,sname,a.course_id,a.number,b.course_id,b.number from score a, score b,student where a.student_id=b.student_id and a.student_id=student.sid and a.course_id=1 and b.course_id=2 and a.number>b.number;
10、查詢有課程成績小於60分的同學的學號、姓名；
select student_id,sname from score,student where student_id=student.sid and number<60 group by student_id;
11、查詢沒有學全所有課的同學的學號、姓名；
select student.sid,sname from student left join score on student.sid=student_id group by student.sid having count(*)<(select count(*) from course);
12、