WITH calc AS (
  SELECT
    MONTHLY,
    NEW_MRR,
    EXPANSION_MRR,
    DOWNGRADE_MRR,
    CHURN_MRR,
    LAG(NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR, 1, 0) OVER (ORDER BY MONTHLY) AS LAST_MONTH_MRR
  FROM MRR_DATA
)
SELECT
  MONTHLY AS YM,
  LAST_MONTH_MRR + NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR AS MRR
FROM calc
ORDER BY YM;