WITH t1 AS (SELECT MONTHLY
                 , NEW_MRR + mrr.EXPANSION_MRR - mrr.DOWNGRADE_MRR - mrr.CHURN_MRR AS x
            FROM MRR_DATA mrr)
   , month AS (SELECT DISTINCT monthly
               FROM MRR_DATA
               UNION ALL
               SELECT DATE(MAX(monthly), '+1 month')
               FROM MRR_DATA)
SELECT m.MONTHLY YM, IFNULL(SUM(t1.x), 0) MRR
FROM month m
     LEFT JOIN t1 ON t1.MONTHLY < m.MONTHLY
GROUP BY m.MONTHLY
order by m.monthly;