题解 | 0级用户高难度试卷的平均用时和平均得分

select uid,
cast(avg(score_took) as signed) as avg_score,
round(avg(time_took),1) as avg_time_took
from 
(select ui.uid,er.exam_id,
ifnull(timestampdiff(minute,start_time,submit_time),duration) as time_took,
ifnull(score,0) as score_took
from exam_record er
join examination_info ei on er.exam_id=ei.exam_id
join user_info ui on er.uid = ui.uid
where level =0
and difficulty = 'hard') a
group  by uid

先在a层用ifnull解决空值，再在外层聚合计算