题解 | #0级用户高难度试卷的平均用时和平均得分#

select ui.uid, 
    round(sum(if(score is null, 0,score))/count(*)) avg_score,
    round(sum(if (submit_time is null, duration, timestampdiff(minute, start_time, submit_time) ))/count(*),1) avg_time_took
from user_info ui left join exam_record er
on ui.uid = er.uid
join examination_info ei
on ei.exam_id = er.exam_id
where level = 0
  and difficulty = 'hard'
group by ui.uid