题解 | #统计每个学校的答过题的用户的平均答题数#

select
    university,
	#注意distinct qpd.device_id要指明对qpd或up表中的device_id去重，否则无法明确是哪个device_id
    round(count(question_id) / count(distinct qpd.device_id), 4) as avg_answer_cnt
from
    user_profile as up
    inner join question_practice_detail qpd on up.device_id = qpd.device_id
group by
    university;