SELECT
    MONTHLY AS YM,
    SUM(tmp_mrr) OVER(ORDER BY MONTHLY) AS MRR
FROM (        
    SELECT
        *,
        tmp_mrr + IFNULL(LAG(tmp_mrr) OVER(ORDER BY MONTHLY), 0) AS tmp2
    FROM (
        SELECT
            *,
            IFNULL(LAG(NEW_MRR) OVER(ORDER BY MONTHLY), 0) + IFNULL(LAG(EXPANSION_MRR) OVER(ORDER BY MONTHLY), 0) - IFNULL(LAG(DOWNGRADE_MRR) OVER(ORDER BY MONTHLY), 0) - IFNULL(LAG(CHURN_MRR) OVER(ORDER BY MONTHLY), 0) AS tmp_mrr
        FROM
            MRR_DATA
    )
)
ORDER BY
    MONTHLY ASC