전월이월 데이터를 거래처당 한번씩만 더해주고싶습니다.

첫번째사진을 보시면 거래처-DDATE마다 한번씩 더해주고있는데 

두번째사진처럼 거래처당 가장첫번째 잔액에만 전월이월데이터를 더해주고싶습니다.

IF CASE문으로 시도해보고있는데 잘 생각이 안나서 질문드립니다

고견 부탁드리겠습니다.

(현재 쿼리)

WITH COST_TAB AS (
    SELECT 1 SEQ, '청과물상회' SHOP, '2020-01-01' DDATE, 300000 TT_COST
    UNION ALL
    SELECT 2 SEQ, '청과물상회' SHOP, '2020-01-01' DDATE, 180000 TT_COST
    UNION ALL
	SELECT 3 SEQ, '청과물상회' SHOP, '2020-02-01' DDATE, 200000 TT_COST
    UNION ALL
    SELECT 4 SEQ, '청과물상회' SHOP, '2020-02-01' DDATE, 80000 TT_COST
    UNION ALL
    SELECT 5 SEQ, '미용실' SHOP, '2020-01-01' DDATE, 225000 TT_COST
	UNION ALL
	SELECT 6 SEQ, '미용실' SHOP, '2020-01-01' DDATE, 175000 TT_COST
	UNION ALL
	SELECT 7 SEQ, '미용실' SHOP, '2020-02-01' DDATE, 125000 TT_COST
	UNION ALL
	SELECT 8 SEQ, '미용실' SHOP, '2020-02-01' DDATE, 275000 TT_COST
), BAL_TAB AS (
    SELECT 1 SEQ, '청과물상회' SHOP,'[전월이월]' DDATE, 10000 BALANCE
	UNION ALL
	SELECT 2 SEQ, '미용실' SHOP,'[전월이월]' DDATE, 20000 BALANCE
)
SELECT *
FROM(
SELECT  SEQ, SHOP, DDATE, 0 AS TT_COST, BALANCE
FROM BAL_TAB

UNION ALL

SELECT CT.SEQ, CT.SHOP, CT.DDATE, CT.TT_COST, BT.BALANCE + CT.SUM_TT AS BALANE
FROM  BAL_TAB BT ,(
    SELECT SEQ, SHOP, DDATE, TT_COST ,SUM(TT_COST)OVER(PARTITION BY SHOP,DDATE ORDER BY SEQ) SUM_TT
FROM COST_TAB) CT
WHERE CT.SHOP = BT.SHOP
) DD
ORDER BY SHOP , DDATE , BALANCE

IF DDATE == '2020-10-01' THEN BT.BALANCE + CT.SUM_TT AS BALANE  이렇게 값을 정해놓지 않아야합니다. DDATE라는건 변하는 값이라...ㅠ