题解 | #统计每个用户的平均刷题数#

select university,difficult_level,round(count(b.question_id)/count(distinct(b.device_id)),4) as avg_answer_cnt 
from user_profile a
left join question_practice_detail b on a.device_id = b.device_id
left join question_detail c on b.question_id = c.question_id
group by university,difficult_level
having university = '山东大学'

第一次最后一个having没写出来，加上后就通过了，没有写a。 b。 c。 但是count里面需写