SELECT VAL 
     , TO_CHAR(ADD_MONTHS(TO_DATE(START_DT,'YYYY-MM-DD') , LV -1),'YYYY-MM') YYYYMM
     , LV
     , MONTHS_BETWEEN(TO_DATE(SUBSTR(END_DT,1,7),'YYYY-MM') , TO_DATE(SUBSTR(START_DT,1,7),'YYYY-MM'))      
FROM
(
SELECT 'A' VAL, '2010-01-20' START_DT, '2012-08-30' END_DT FROM DUAL UNION ALL
SELECT 'B' , '2011-02-18' , '2012-07-11' FROM DUAL 
) A , (SELECT LEVEL LV FROM DUAL CONNECT BY LEVEL <= 100 ) B
WHERE MONTHS_BETWEEN(TO_DATE(SUBSTR(END_DT,1,7),'YYYY-MM') , TO_DATE(SUBSTR(START_DT,1,7),'YYYY-MM')) >= B.LV - 1 
ORDER BY 1 , 2

제품코드가 2개가 아닌 유동적이라면

WITH T AS (
  SELECT 'A' AS 제품코드, '2010-01-20' AS 판매시작일, '2012-08-30' AS 판매종료일 FROM DUAL UNION ALL
  SELECT 'B','2011-02-18','2012-07-11' FROM DUAL UNION ALL
  SELECT 'C','2012-02-18','2012-12-11' FROM DUAL 
)
SELECT
  T2.제품코드, TO_CHAR(TO_DATE(기준일), 'YYYY-MM') AS 기준일
FROM (
  SELECT 
    TO_CHAR(ADD_MONTHS(LAST_DAY(sd), LEVEL - 1), 'YYYY-MM-DD') AS 기준일
  FROM (
    SELECT MIN(판매시작일) AS sd, MAX(판매종료일) AS ed FROM T
  )
  CONNECT BY
    ADD_MONTHS(LAST_DAY(sd), LEVEL - 1) <= LAST_DAY(ed)
) T1, (SELECT 제품코드, 판매시작일, TO_CHAR(LAST_DAY(판매종료일), 'YYYY-MM-DD') AS 판매종료일 FROM T) T2
WHERE
  1 = CASE WHEN(기준일 >= 판매시작일 AND 기준일 <= 판매종료일) THEN 1 ELSE 0 END 
ORDER BY
  제품코드, 기준일
;

WITH t(code, sdt, edt) AS
(
SELECT 제품코드
     , trunc(판매시작일,'MM')
     , trunc(판매종료일,'MM')
  FROM 제품정보
  UNION ALL
SELECT code
     , add_months(sdt,1)
     , edt 
  FROM t
WHERE add_months(sdt,1) <= edt
)
SELECT code, to_char(sdt,'YYYY-MM')
  FROM t
 ORDER BY code, sdt;