WITH RECURSIVE calendar(MONTHLY) AS (
    SELECT
        -- ここが始まりの日付
        DATE((SELECT MIN(MONTHLY) FROM MRR_DATA)) AS MONTHLY
    UNION ALL
    -- UNION ALL 以下が再帰処理部分
    SELECT
        DATE(MONTHLY, '+1 month') AS MONTHLY
    FROM calendar
    WHERE
        -- ここが終わりの日付
        MONTHLY < DATE((SELECT MAX(MONTHLY) FROM MRR_DATA))
)

SELECT A.MONTHLY AS "YM",
(SELECT COALESCE(SUM(B.NEW_MRR + B.EXPANSION_MRR - B.DOWNGRADE_MRR - B.CHURN_MRR),0) FROM MRR_DATA AS B WHERE A.MONTHLY > B.MONTHLY) AS "MRR" FROM calendar AS A ORDER BY YM;