없는 시간도 나오게 하려면
전체 시간 집합을 이용해
아우터 조인 하셔야 합니다.

WITH code_t AS
(
SELECT LEVEL - 1 hh FROM dual CONNECT BY LEVEL <= 24
)
, data_t AS
(
SELECT 1 hh, 5 cnt FROM dual
UNION ALL SELECT 2, 4 FROM dual
UNION ALL SELECT 3, 3 FROM dual
UNION ALL SELECT 4, 7 FROM dual
)
SELECT a.hh
     , NVL(b.cnt, 0) cnt
  FROM code_t a
     , data_t b
 WHERE a.hh = b.hh(+)
 ORDER BY hh
;