Can you try the following
SELECT A.SHOW_ROOM,SUM(A.ACTUAL_SALES) AS ACTUAL_SALES,SUM(A.PLAN_SALES) AS PLAN_SALES
FROM
(
SELECT NVL(A.SHOW_ROOM,P.SHOW_ROOM) AS SHOW_ROOM,
SUM(A.ACTUAL_SALES) AS ACTUAL_SALES,SUM(P.PLAN_SALES) AS PLAN_SALES
FROM SALES_ACT A
FULL OUTER JOIN SALES_PLAN P ON A.PERIOD = P.PERIOD AND A.SHOW_ROOM = P.SHOW_ROOWM
GROUP BY A.SHOW_ROOM,P.SHOW_ROOM
) A
GROUP BY A.SHOW_ROOM;
Regards,
Nithin