题解 | #0级用户高难度试卷的平均用时和平均得分#

select 
    a.uid
	-- 空值转换
    , round(avg(if(a.score is null, 0, score)), 0) avg_score
    , round(avg(if(a.submit_time is null, b.duration, timestampdiff(minute, a.start_time, a.submit_time))), 1) avg_time_took
from 
    exam_record a
    left join examination_info b
    on a.exam_id = b.exam_id
    left join user_info c
    on a.uid = c.uid
where 
    b.difficulty = 'hard'
    and c.level = 0
group by 
    a.uid