题意：每个学校答题总量 / 学生

解题思路：用大学进行分组，统计question_practice_detail组内总数量即为每个学校答题总数
再计算出学生的个数，学生个数就是答题的同学去重后的数量

select university,
round(count(*) / count(distinct(a.device_id))
      ,4) as avg_answer_cnt
from question_practice_detail as a
left join user_profile as b
on a.device_id = b.device_id
group by university
order by university

Created by WangChao