题解 | #统计每个学校各难度的用户平均刷题数#

运营想要计算一些参加了答题的不同学校、不同难度的用户平均答题量，请你写SQL取出相应数据
分组     a.university,c.difficult_level
平均答题量  count(b.question_id)/count(distinct(b.device_id)) avg_answer_cnt
select
    a.university,
    c.difficult_level,
    count(b.question_id) / count(distinct (b.device_id)) avg_answer_cnt
from
    user_profile a
    inner join question_practice_detail b on a.device_id = b.device_id
    inner join question_detail c on b.question_id = c.question_id
group by
    a.university,
    c.difficult_level