WITH numseq AS (
  SELECT 
    MONTHLY,
    NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR AS num
  FROM 
    MRR_DATA
),
cum AS (
  SELECT
    MONTHLY AS YM,
    SUM(num) OVER (ORDER BY MONTHLY) AS MRR
  FROM 
    numseq
),
last_month_added AS (
  SELECT YM, MRR FROM cum
  UNION ALL
  SELECT date(MAX(YM), '+1 month') AS YM, 0 AS MRR FROM cum
)
SELECT 
  YM,
  COALESCE(LAG(MRR) OVER (ORDER BY YM), 0) AS MRR
FROM 
  last_month_added
ORDER BY 
  YM;