题解 | 所有用户对高难度题目的刷题数

select up.device_id,cast(sum(case when difficult_level='hard' then 1 else 0 end) as signed) question_cnt from
user_profile up left join question_practice_detail qpd on up.device_id=qpd.device_id left join question_detail qd on qpd.question_id=qd.question_id
group by up.device_id
order by question_cnt

cast cast函数修改数据类型 signed整数