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(case when score is null then 0 else score end)) as avg_score,
round(avg(case when score is null then duration else timestampdiff(minute,start_time,submit_time) end),1) as avg_time_took
from exam_record join examination_info using(exam_id)
                 join user_info        using(uid)
where level = 0 and difficulty = 'hard'
group by uid

这个题主要的就是case when的使用和格式调整