SELECT up.device_id AS "device_id", up.university AS "university", COUNT(if(month(qpd.date) = 8,1,null)) AS 'question_cnt', # 对于count,任何值都会计数,只有null值不会计数 COUNT(if(qpd.result = 'right' AND month(qpd.date) = 8 ,1,null)) AS 'right_question_cnt' FROM user_profile up LEFT JOIN ques...