select student.sid,student.sname from student right join (select New.student_id as ID,count(New.course_id) as Num from (select * from score where course_id=1 or course_id=2) as New group by New.student_id having Num=2) as A on student.sid=A.ID;

（1）查询出小多老师的课程数量
select count(course.cid) as Num from course left join teacher on course.teacher_id=teacher.tid where teacher.tname='小多' group by teacher.tid;（2）查询出上过小多老师课的学生ID和该学生上过小多老师的课程数量
select score.student_id,count(score.course_id) from score where score.course_id in (select cid from teacher left join course on teacher.tid=course.teacher_id where teacher.tname='小多') group by score.student_id（3）连表
select student.sid,student.sname from student right join (select B.ID_B as ID_A from (select score.student_id as ID_B,count(score.course_id) as NumB from score where score.course_id in (select cid from teacher left join course on teacher.tid=course.teacher_id where teacher.tname='小多') group by score.student_id) as B left join (select count(course.cid) as NumC from course left join teacher on course.teacher_id=teacher.tid where teacher.tname='小多' group by teacher.tid) as C on B.NumB=C.NumC) as A on student.sid=A.ID_A;

select student.sid,student.sname from student right join (select B.student_id as ID_A from (select student_id,number from score where course_id=1) as B left join (select student_id,number from score where course_id=2) as C on B.student_id=C.student_id where B.number > C.number) as A on student.sid=A.ID_A;

select sid,sname from student where sid in (select student_id from score where number<60);

select sid,sname from student where sid not in (select student_id from score group by student_id having count(course_id)=3);

select student.sid,student.sname from student right join (select student_id from score where student_id !=1 and course_id in (select course_id from score where student_id=1) group by student_id) as A on student.sid=A.student_id;

select student.sid,student.sname from student right join (select student_id,count(course_id) from score where student_id !=2 and course_id in (select course_id from score where student_id=2) group by student_id having count(course_id)=(select count(course_id) from score where student_id=2 group by student_id)) as A on student.sid=A.student_id;

delete from score where course_id = (select cid from course left join teacher on course.teacher_id=teacher.tid where teacher.tname='叶平');