学生表 A1（sid,sname） 课程表A2(cid,cno) 成绩表A3(scid,sid,cid,defen)
查询 课程标号cno2的成绩大于课程标号cno1的成绩的学生名
------解决方案--------------------
select t5.sname
from a3 t1,a3 t2,a2 t3,a2 t4,a1 t5
where t1.sid=t2.sid and t1.cid=t3.cid and t2.cid=t4.cid and t1.sid=t5.sid
where t3.cno='cno1' and t4.cno='cno2' and t1.defen>t2.defen