SELECT
    DATE(MONTHLY, '+1 month') AS YM,
    SUM(MRR_BASE) OVER (ORDER BY MONTHLY ROWS UNBOUNDED PRECEDING) AS MRR
FROM (
    SELECT
        MONTHLY,
        (COALESCE(LAG(NEW_MRR, 1) OVER (ORDER BY MONTHLY), 0)
         + COALESCE(LAG(EXPANSION_MRR, 1) OVER (ORDER BY MONTHLY), 0)
         - COALESCE(LAG(DOWNGRADE_MRR, 1) OVER (ORDER BY MONTHLY), 0)
         - COALESCE(LAG(CHURN_MRR, 1) OVER (ORDER BY MONTHLY), 0)
        ) MRR_BASE
    FROM
        MRR_DATA
)
ORDER BY
    MONTHLY
;