WITH D AS (SELECT MIN(MONTHLY) MINMO, MAX(MONTHLY) MAXMO FROM MRR_DATA),
calendar AS (
    SELECT MINMO AS calendar_date FROM D
    UNION ALL
    SELECT
        DATE(calendar_date, '+1 month') AS calendar_date
    FROM calendar
    WHERE
        calendar_date < (SELECT MAXMO FROM D)
),
T AS (SELECT C.calendar_date MONTHLY, ifnull(NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR, 0) MRR, ROW_NUMBER() OVER(ORDER BY C.calendar_date) RAN FROM calendar c left join MRR_DATA M on C.calendar_date = M.MONTHLY),
 M AS (SELECT MONTHLY YM, lag(MRR, 1, 0) OVER (ORDER BY RAN) MRR, RAN FROM T)
SELECT M1.YM, (SELECT SUM(MRR) FROM M M2 WHERE M2.RAN <= M1.RAN) MRR
FROM M M1
ORDER BY 1