SELECT DISTINCT ON (t._id) t._id,
                   t._date
FROM
  (SELECT schedule_lesson.id AS _id,
          latest_scehedule_lesson.date_ AS _date
   FROM schedule_lesson
   JOIN studytimecall ON schedule_lesson.studycall_id = studytimecall.id
   JOIN
     (SELECT schedule_lesson.id AS id,
             schedule_lesson.discipline_group_id AS discipline_group_id,
             schedule_lesson.date AS date_,
             studytimecall.begin AS begin_
      FROM schedule_lesson
      JOIN studytimecall ON schedule_lesson.studycall_id = studytimecall.id) AS latest_schedule_lesson ON schedule_lesson.discipline_group_id=latest_schedule_lesson.discipline_group_id
   AND schedule_lesson.id <> latest_schedule_lesson.id
   AND (schedule_lesson.date < latest_schedule_lesson.date_
        OR (schedule_lesson.date = latest_schedule_lesson.date_
            AND studytimecall.begin < latest_schedule_lesson.begin_))
   ORDER BY schedule_lesson.id,
            latest_schedule_lesson.date_ DESC, latest_schedule_lesson.begin_ DESC) AS t