특정 중간 항목을 선택한 MySQL 중첩 집계 쿼리 이건 내 거야activities테이블. activities +----+---------+----------+-----------------+ | id | user_id | activity | log_time | +----+---------+----------+-----------------+ | 6 | 1 | start | 12 Oct, 1000hrs | | 2 | 1 | task | 12 Oct, 1010hrs | | 7 | 1 | task | 12 Oct, 1040hrs | | 3 | 1 | start | 12 Oct, 1600hrs | | 1 | 1 | task | 12 Oct, 1610hrs | | 9 | 1 | start | 14 Oct, 08..