select difficult_level, avg(if(qpd.result='right', 1, 0)) as correct_rate #sum(if(qpd.result='right', 1, 0)) / count(qpd.question_id) as correct_rate #count(if(qpd.result='right', 1, null)) / count(qpd.question_id) as correct_rate from user_profile as up join question_practice_detail as...