select st.name na,co.NAME name,sc.score from scores sc
left join students st on sc.sid = st.id
left join courses co on co.id=sc.cid

共9门课程，获取成绩未填满的学生：

select t.na,count(*) from

(select st.name na,co.NAME name,sc.score from scores sc
left join students st on sc.sid = st.id
left join courses co on co.id=sc.cid) t

group by t.na having count(*)<9;