WITH t AS
(
SELECT 50020 id, '20160131230151' reg_date FROM dual
UNION ALL SELECT 50021, '20160201124530' FROM dual
UNION ALL SELECT 50022, '20160205190811' FROM dual
UNION ALL SELECT 50023, '20160211161821' FROM dual
UNION ALL SELECT 50024, '20160215220104' FROM dual
UNION ALL SELECT 50025, '20160225140224' FROM dual
UNION ALL SELECT 50026, '20160305094255' FROM dual
)
SELECT a.시간
     , NVL(b.월, 0) 월
     , NVL(b.화, 0) 화
     , NVL(b.수, 0) 수
     , NVL(b.목, 0) 목
     , NVL(b.금, 0) 금
     , NVL(b.토, 0) 토
     , NVL(b.일, 0) 일
  FROM (SELECT LPAD(LEVEL-1, 2, '0') hh24
             , LPAD(LEVEL-1, 2, '0') ||'~'|| LPAD(LEVEL, 2, '0') 시간
          FROM dual
         CONNECT BY LEVEL <= 24
        ) a
     , (SELECT hh24
             , COUNT(DECODE(d, '2', 1)) 월
             , COUNT(DECODE(d, '3', 1)) 화
             , COUNT(DECODE(d, '4', 1)) 수
             , COUNT(DECODE(d, '5', 1)) 목
             , COUNT(DECODE(d, '6', 1)) 금
             , COUNT(DECODE(d, '7', 1)) 토
             , COUNT(DECODE(d, '1', 1)) 일
          FROM (SELECT SUBSTR(reg_date, 9, 2) hh24
                     , TO_CHAR(TO_DATE(reg_date, 'yyyymmddhh24miss'), 'd') d
                  FROM t
                -- WHERE 검색기간
                )
         GROUP BY hh24
        ) b
 WHERE a.hh24 = b.hh24(+)
 ORDER BY a.hh24, a.시간
;

와 엄청 머리 싸매고 있었는데 한 방에 해결됐네요.

마농 님 감사합니다.