题解 | #0级用户高难度试卷的平均用时和平均得分#

select 
    uid,
    round(avg(if(score is null ,0,score)),0) as avg_score,
    round(avg(if(submit_time is null,
                 (select duration from examination_info ei where er.exam_id=ei.exam_id),
                 TIMESTAMPDIFF(MINUTE,start_time,submit_time))),1)as avg_time_took
from exam_record er
where uid in (select uid from user_info where level =0)
    and exam_id in (select exam_id from examination_info where difficulty='hard')
group by uid