select   b.tag,   concat(     round(       avg(         case           when timestampdiff(second, start_time, end_time) ...

select   university,   difficult_level,   sum(date) / count(u.device_id) avg_answer_cnt from   (     select       *     from ...

select   university,   difficult_level,   sum(date) / count(university) from   user_profile a,   (     select       device_id, &nbs...

select   university,   sum(date) / count(a.device_id) from   user_profile a,   (     select       device_id,       CO...

select gender, university, COUNT(id) user_num, avg(active_days_within_30) avg_active_day, avg(question_cnt) avg_question_cnt from user_profile group by university, gender;