题解 | #统计每个学校的答过题的用户的平均答题数#

select 
    university, 
    round(count(question_id)/count(distinct q.device_id), 4) as avg_answer_cnt
from 
    user_profile u,  
    question_practice_detail q 
where 
    u.device_id = q.device_id 
group by university
order by university asc

round (列， 4) 保留4位

distinct 去除重复的字段

用回答的题目数量除以用户数量 (一个设备对应多个题，要去重)