第二个怎么写

随便写写, 仅供参考:
select cl.className, co.course, avg(sc.score)
from student st
inner join score sc on sc.stuID = st.stuID
inner join class cl on c.classID = st.classID
inner join course co on co.courseID = sc.courseID
group by cl.className, co.course

MySQL用分组就行了