WITH calc AS (
  SELECT
    MONTHLY,
    NEW_MRR,
    EXPANSION_MRR,
    DOWNGRADE_MRR,
    CHURN_MRR,
    COALESCE(LAG(NEW_MRR) OVER (ORDER BY MONTHLY), 0) + 
    COALESCE(LAG(EXPANSION_MRR) OVER (ORDER BY MONTHLY), 0) - 
    COALESCE(LAG(DOWNGRADE_MRR) OVER (ORDER BY MONTHLY), 0) - 
    COALESCE(LAG(CHURN_MRR) OVER (ORDER BY MONTHLY), 0) 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;