题解 | #所有用户对高难度题目的刷题数#

select
    up.device_id,
    convert(sum(if(a.difficult_level='hard',1,0)),signed) as question_cnt
from user_profile as up
left join
(
select  
    qpd.device_id,
    qpd.question_id,
    qd.difficult_level
from question_practice_detail as qpd
left join question_detail as qd
on qpd.question_id=qd.question_id
) as a
on a.device_id=up.device_id
group by device_id
order by question_cnt