참고 하시기 바랍니다.

WITH T AS(
SELECT 1 AS C1, TO_DATE('20140510', 'YYYYMMDD') AS C2, TO_DATE('99991231', 'YYYYMMDD') AS C3 FROM DUAL UNION ALL
SELECT 2 AS C1, TO_DATE('20140525', 'YYYYMMDD') AS C2, TO_DATE('99991231', 'YYYYMMDD') AS C3 FROM DUAL UNION ALL
SELECT 3 AS C1, TO_DATE('20140710', 'YYYYMMDD') AS C2, TO_DATE('20150324', 'YYYYMMDD') AS C3 FROM DUAL UNION ALL
SELECT 4 AS C1, TO_DATE('20140510', 'YYYYMMDD') AS C2, TO_DATE('20150424', 'YYYYMMDD') AS C3 FROM DUAL)
SELECT A.C1, A.C2, A.C3, COUNT(C.TT) AS CNT
  FROM ( SELECT A.C1, A.C2, A.C3,
                TO_CHAR(GREATEST(A.C2, B.C1), 'YYYYMM') AS ST_MON, TO_CHAR(LEAST(A.C3, B.C2), 'YYYYMM') AS END_MON
           FROM T A,
                ( SELECT ADD_MONTHS(TRUNC(SYSDATE, 'DD'), -6) + 1 AS C1, TRUNC(SYSDATE, 'DD') AS C2 FROM DUAL ) B
          WHERE A.C2 <= B.C2(+)
            AND A.C3 >= B.C1(+) ) A,
       ( SELECT TO_CHAR(ADD_MONTHS(TRUNC(SYSDATE, 'MM'), (LEVEL - 1 )* -1 ), 'YYYYMM') AS TT
           FROM DUAL
        CONNECT BY LEVEL <= 7 ) C
 WHERE C.TT(+) BETWEEN A.ST_MON AND A.END_MON
 GROUP BY A.C1, A.C2, A.C3
 ORDER BY C1

WITH TT AS(
    SELECT 1 AS C1, '20140510' AS C2, '99991231' AS C3 FROM DUAL UNION ALL
    SELECT 2 AS C1, '20140525' AS C2, '99991231' AS C3 FROM DUAL UNION ALL
    SELECT 3 AS C1, '20140610' AS C2, '99991231' AS C3 FROM DUAL UNION ALL
    SELECT 4 AS C1, '20140710' AS C2, '20150324' AS C3 FROM DUAL UNION ALL
    SELECT 5 AS C1, '20140710' AS C2, '20150420' AS C3 FROM DUAL UNION ALL
    SELECT 6 AS C1, '20150110' AS C2, '99991231' AS C3 FROM DUAL UNION ALL
    SELECT 7 AS C1, '20150110' AS C2, '20150324' AS C3 FROM DUAL UNION ALL
    SELECT 8 AS C1, '20150120' AS C2, '20150328' AS C3 FROM DUAL UNION ALL
    SELECT 9 AS C1, '20150301' AS C2, '99991231' AS C3 FROM DUAL UNION ALL
    SELECT 10 AS C1, '20150410' AS C2, '20150924' AS C3 FROM DUAL UNION ALL
    SELECT 11 AS C1, '20140510' AS C2, '00000000' AS C3 FROM DUAL UNION ALL
    SELECT 12 AS C1, '20140520' AS C2, '00000000' AS C3 FROM DUAL
)
SELECT C1, C2, C3
     , LEAST(CEIL(6-MONTHS_BETWEEN(TRUNC(SYSDATE,'MM'), TO_DATE(DECODE(C3,'00000000',NULL,C3),'YYYYMMDD'))),6) 산출월수
FROM TT
WHERE TO_DATE(DECODE(C3,'00000000',NULL,C3),'YYYYMMDD') > ADD_MONTHS(SYSDATE,-6)
;

SELECT 순번, 시작일자, 종료일자
     , MONTHS_BETWEEN
       ( TO_DATE(SUBSTR(LEAST   (종료일자, edt), 1, 6), 'yyyymm')
       , TO_DATE(SUBSTR(GREATEST(시작일자, sdt), 1, 6), 'yyyymm')
       ) + 1 AS cnt
  FROM t
     , (SELECT TO_CHAR(ADD_MONTHS(sysdate, -6) + 1, 'yyyymmdd') sdt
             , TO_CHAR(sysdate, 'yyyymmdd') edt
          FROM dual
        )
 WHERE 시작일자 <= edt
   AND 종료일자 >= sdt
 ORDER BY 순번
;