SELECT er.uid, ROUND(SUM(IF(score IS NULL,0,er.score))/COUNT(*),0) AS avg_score, ROUND(SUM(IF(er.submit_time is NULL,t2.duration,TIMESTAMPDIFF(MINUTE,er.start_time,er.submit_time)))/COUNT(*),1) AS avg_time_took FROM exam_record er JOIN ( SELECT uid FROM user_info WHERE level = 0 ) AS t1 ON er.uid = ...