只需要注意分组加上 去不去重
统计每个学校各难度的用户平均刷题数
https://www.nowcoder.com/practice/5400df085a034f88b2e17941ab338ee8
select
university,difficult_level,
count(qpd.question_id)/count( distinct qpd.device_id) as avg_answer_cnt
from
user_profile as up
join question_practice_detail as qpd on up.device_id = qpd.device_id
join question_detail as qd on qd.question_id = qpd.question_id
group by university,difficult_level
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