题解 | #统计每个用户的平均刷题数#

select
    '山东大学',
    difficult_level,
    count(up.device_id) / count(distinct up.device_id)
from
    question_practice_detail as qpd
    inner join user_profile as up on qpd.device_id = up.device_id
    and up.university = '山东大学'
    inner join question_detail as qd on qpd.question_id = qd.question_id
group by
    qd.difficult_level

inner join user_profile as up on qpd.device_id = up.device_id

and up.university = '山东大学'

inner join 添加表格的时候自动删除含有空白值的行

and up.university = '山东大学' 在使用inner，left，right的时候可以使用and增减筛选条件