1两表字段一致内连接 连接两个表，且一张表的一个数据大于另一张表的一个数据
select s.*,sc1.s_score as score_1,sc2.s_score as score_2
from student s
inner join(select * from score where cid=1)sc1
on s.s_id=sc1.s_id
inner join(select * from score where cid=2)sc2
on s.s_id=sc2.s_id
where sc1.s_score >sc2.s_score
