SQL22、统计每个学校的答过题的用户的平均答题数

思路：使用表连接将有关联的表合并为大表。并用分组将重复的学校数据合并。同时在计算平均答题数时用所有题目数除以用户数目

代码解释：对连接表进行分组并取出数据

SELECT 
    u.university,
    COUNT(q.question_id) / COUNT(DISTINCT q.device_id) AS avg_answer_cnt
FROM 
    user_profile AS u
    INNER JOIN question_practice_detail AS q
    ON u.device_id = q.device_id
GROUP BY 
    u.university 

题目：https://www.nowcoder.com/practice/88aa923a9a674253b861a8fa56bac8e5?tpId=199&tags=&title=&difficulty=0&judgeStatus=0&rp=0&sourceUrl=%2Fexam%2Foj%3Fpage%3D1%26tab%3DSQL%25E7%25AF%2587%26topicId%3D199