WITH t1 AS (
    SELECT
      MONTHLY AS YM,
      LAG((NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR)) OVER (ORDER BY MONTHLY) AS tmp1
    FROM
      MRR_DATA
    ORDER BY
      MONTHLY
), t2 AS (
    SELECT
      YM,
      SUM(tmp1) OVER (ORDER BY YM) AS tmp2
    FROM
      t1
)

SELECT
  YM,
  CASE tmp2
    WHEN null THEN 0
    ELSE tmp2
  END
FROM t2