with t as (
select to_date('2011-01-05', 'yyyy-mm-dd') sdt, to_date('2011-09-27', 'yyyy-mm-dd') edt from dual
)
select to_char(case when level = 1 then sdt else trunc(add_months(sdt, level - 1), 'mm') end, 'yyyy-mm-dd') sdt
     , to_char(case when level = trunc(months_between(edt, sdt), 0) + 1 then edt else last_day(add_months(sdt, level - 1)) end, 'yyyy-mm-dd') edt
  from t
 connect by level <= trunc(months_between(edt, sdt), 0) + 1

[ 결과 ]

sdt        edt

-----------------------

2011-01-05 2011-01-31
2011-02-01 2011-02-28
2011-03-01 2011-03-31
2011-04-01 2011-04-30
2011-05-01 2011-05-31
2011-06-01 2011-06-30
2011-07-01 2011-07-31
2011-08-01 2011-08-31
2011-09-01 2011-09-27

WITH t AS
(
SELECT TO_DATE('2011-01-05', 'yyyy-mm-dd') sdt
     , TO_DATE('2011-09-27', 'yyyy-mm-dd') edt
  FROM dual
)
SELECT GREATEST(ADD_MONTHS(TRUNC(sdt, 'mm'), level - 1), sdt) sdt
     , LEAST(   ADD_MONTHS(TRUNC(sdt, 'mm'), level) - 1, edt) edt
  FROM t
 CONNECT BY LEVEL <= MONTHS_BETWEEN(TRUNC(edt, 'mm'), TRUNC(sdt, 'mm')) + 1
;