题解 | #0级用户高难度试卷的平均用时和平均得分#

思路：①第二、第三张表连接，选出hard试卷对应行，然后用if处理null，即如果交卷时间是null则赋值duration和0分；
②根据uid分组，计算平均分和平均用时

select uid, round(avg(sc),0), round(avg(ti),1)
from
(
select uid, exam_id,
if(submit_time is null,duration,timestampdiff(minute,start_time,submit_time)) as ti,
if(score is null,0,score) as sc
from 
exam_record left join examination_info using(exam_id)
where difficulty='hard') as tb1
left join user_info using(uid)
where level=0
group by uid