题解 | #统计每个学校的答过题的用户的平均答题数#

select university,
count(question_id) / count(distinct qpd.device_id) as avg_answer_cnt #答题总数除以人总数(人总数去重)
from question_practice_detail as qpd inner join user_profile as up   #关联查询
on qpd.device_id=up.device_id                                        #两表device_id相等
group by university                                                  分组