--悔しいけど答えを参考に...

WITH master_table AS (
--最も古い年月のMRRを0とする
SELECT
 (SELECT MIN(MONTHLY) FROM MRR_DATA) AS YM,
 0 AS MRR
 UNION ALL
 --一か月ごとにカウントアップする
SELECT
 DATE(YM,'+1 month'),
  MRR + (SELECT NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR FROM MRR_DATA WHERE MONTHLY = YM)
FROM
 master_table
WHERE
 YM < DATE((SELECT MAX(MONTHLY) FROM MRR_DATA),'+1 month')

)

SELECT
 *
FROM
 master_table
ORDER BY
 YM