SELECT uu university,dl difficult_level,avg(cc) avg_answer_cnt
FROM(SELECT u.university uu,u.device_id,qpd_qd.d_lev dl,count(*) cc
FROM user_profile u
JOIN (
SELECT qpd.device_id d_id,qpd.question_id q_id,qd.difficult_level d_lev
FROM question_practice_detail qpd
JOIN question_detail qd
ON qpd.question_id = qd.question_id
) qpd_qd
ON u.device_id = qpd_qd.d_id
GROUP BY u.device_id,qpd_qd.d_lev) u_qpd_qd
GROUP BY uu,dl;
先理清楚思路再解决问题,上来就写代码改来改去好崩溃!