CASE WHEN B.XARM_TEMP01 IS NULL THEN '0' ELSE XARM_TEMP01 END

-- B.XARM_TEMP01 =  NULL  로 해보세요 mssql경우 is null로 하면 에러가 떨어진는데...

GROUP BY 제거하세요

SELECT
    A.*,
    (
        SELECT VALUE(SUM(INTEGER(B.XARM_TEMP01)), 0) AS XARM_TEMP01
        FROM   XDDB2.HGAA_GASCXARM AS B
        WHERE  B.XARM_TEMP01 = A.XARM_TEMP01
        GROUP BY B.XARM_TEMP01
    )
FROM XDDB2.HGAA_GASCXARM AS A