# 要使用timestampdiff()函数来计算时间差
select er.uid, 
round(avg(ifnull(score,0))) avg_score, 
round(avg(ifnull(timestampdiff(minute, start_time, submit_time),duration)),1) avg_time_took
from exam_record er
left join examination_info ei
on er.exam_id=ei.exam_id
left join user_info ui
on er.uid=ui.uid
where level=0 and difficulty='hard'
group by er.uid;