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 question_practice_detail.device_id) as avg_answer_cnt
    from user_profile
    inner join question_practice_detail
    on user_profile.device_id = question_practice_detail.device_id
    group by university

1. 外连接：inner join 只显示有效数据
2. 每个学校：group by university
3. 平均答题数 = 总答题数 / 总人数