select uid,round(avg(score),0) avg_score,round(avg(timedelta),1) avg_time_took from user_info u_i left join ( select uid,exam_id,if(submit_time is null,60,minute(timediff(start_time,submit_time))) timedelta, if(score is null,0,score) score from exam_record e_r left join examination_info e_i using(exam_id) where difficulty='hard' )t1 using(uid) where level=0 group by uid; 请问这样为啥不行呀
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