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