SELECT a.hh24mi
     , NVL(b.v, 0) v
  FROM (-- 1. 기준 집합
        SELECT TO_CHAR(TO_DATE((LEVEL-1) *20*60, 'sssss'), 'hh24:mi') hh24mi
             , (LEVEL-1) * 20 mi
          FROM dual
         CONNECT BY LEVEL <= 24 * 3
        ) a
     , (-- 2. SUM 집합
        SELECT FLOOR(TO_CHAR(dt, 'sssss') /60/20)*20 mi
             , SUM(v) v
          FROM t
         WHERE dt >= TO_DATE(:v_dt, 'yyyymmdd')
           AND dt <  TO_DATE(:v_dt, 'yyyymmdd') + 1
         GROUP BY FLOOR(TO_CHAR(dt, 'sssss') /60/20)*20
        ) b
 WHERE a.mi = b.mi(+)    -- 3. 아우터 조인
 ORDER BY a.hh24mi
;