1.数量错误
avg在不分组的情况下，只显示一组数据
正确为
2.结果不对
select student_id,avg(number) from score where number<60 group by student_id having count(corse_id)>=1;
结果不正确，只是求出了number<60的平均值
select student_id,avg(number) from score where
student_id in (select student_id from score where number < 60 group by student_id having count(corse_id) >=1) group by student_id;
正确