SELECT
     strftime('%Y-%m', A.SALES_DATE) AS YEAR_MONTH
    ,SUM(A.SALES_AMT) AS MONTH_AMT
    ,SUM(B.MONTH_AMT) AS CUML_AMT
FROM
    SALES A
INNER JOIN
    (
    SELECT 
         strftime('%Y-%m', SALES_DATE) AS MONTH
        ,SUM(SALES_AMT) AS MONTH_AMT
    FROM
        SALES
    WHERE
        UPDATED_NO IS NULL
    GROUP BY 
        MONTH
    ) B
ON
    YEAR_MONTH >= B.MONTH
WHERE
    A.UPDATED_NO IS NULL

GROUP BY 
    YEAR_MONTH