postgresql7.1 select user_id as student_name, start_time, end_time from lorsm_student_track where community_id = :community_id and course_id = :man_id and end_time NOTNULL order by start_time desc select user_id as student_name, count(*) as counter, sum(end_time - start_time) as time_spent from lorsm_student_track where community_id = :community_id and course_id = :man_id and end_time NOTNULL group by user_id