题解 | #0级用户高难度试卷的平均用时和平均得分#

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