查询同时'01'课程比'02'分数低或'01'缺考的数据
where isnull（score，0)<b.score
表示无效
where 01.score<0.2.score or 01.score is null
select a.course_no, a.stu_no, a.score , b.course_no, b.stu_no, b.score
from score a, score b
where a.course_no=106 and b.course_no=103
and a.stu_no=b.stu_no
and a.score > b.score or a.score is null