WITH t AS (
  SELECT MIN(MONTHLY) minM
    FROM MRR_DATA
),
t2 AS (
  SELECT MAX(MONTHLY) maxM
    FROM MRR_DATA
)
SELECT M.MONTHLY AS YM
      ,IFNULL((
        SELECT SUM(IFNULL(B.NEW_MRR, 0) + IFNULL(B.EXPANSION_MRR, 0) - IFNULL(B.DOWNGRADE_MRR, 0) - IFNULL(B.CHURN_MRR, 0))
          FROM MRR_DATA B
         WHERE B.MONTHLY < M.MONTHLY
        ), 0) MRR
  FROM MRR_DATA M
 WHERE EXISTS(SELECT 1
                FROM t
               WHERE M.MONTHLY >= t.minM)
UNION
SELECT DATE(maxM,'+1 months') AS YM
      ,IFNULL((
        SELECT SUM(IFNULL(B.NEW_MRR, 0) + IFNULL(B.EXPANSION_MRR, 0) - IFNULL(B.DOWNGRADE_MRR, 0) - IFNULL(B.CHURN_MRR, 0))
          FROM MRR_DATA B
       ), 0) MRR
  FROM t2
ORDER BY YM