题解 | #统计每个用户的平均刷题数#

#选择字段：原生&合成
select
    up.university,
    qd.difficult_level,
    round(count(main.question_id)/count(distinct main.device_id),4) as avg_answer_cnt
#选择表：原生&连接。主表+连接其他表
from
    question_practice_detail as main
    inner join user_profile as up
        on up.device_id = main.device_id
    inner join question_detail as qd
        on qd.question_id = main.question_id
#数据操作段：分组、排序、筛选、取指定行数
where university='山东大学'
group by up.university,qd.difficult_level