where grade <60 group by sid having count(*)>2
)
group by sid;
49、检索“004”课程分数小于60，按分数降序排列的同学学号
select sid from t_score where cid='004'and grade <60 order by grade desc;
50、删除“002”同学的“001”课程的成绩
delete from t_score where sid='001'and cid = '001';