Oracle 18c

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WITH DATAA (orderdate, unit_of_phone_sale) AS
(
SELECT TO_DATE('20190806','YYYYMMDD'), 3000 FROM DUAL UNION ALL
--SELECT TO_DATE('20190704','YYYYMMDD'), 3789 FROM DUAL UNION ALL
SELECT TO_DATE('20190627','YYYYMMDD'), 789 FROM DUAL UNION ALL
SELECT TO_DATE('20190503','YYYYMMDD'), 666 FROM DUAL UNION ALL
SELECT TO_DATE('20190402','YYYYMMDD'), 765 FROM DUAL
)
, 
CTE AS (
    SELECT
        TRUNC(ORDERDATE, 'MONTH') AS ORDER_MONTH,
        SUM(UNIT_OF_PHONE_SALE) AS UNIT_OF_PHONE_SALE
    FROM
        DATAA
    GROUP BY
        TRUNC(ORDERDATE, 'MONTH')
)

SELECT
    TO_CHAR(C.ORDER_MONTH,'YYYYMM') as ORDER_MONTH,
    NVL(C1.UNIT_OF_PHONE_SALE, 0) AS "M-1_Sale",
    NVL(C2.UNIT_OF_PHONE_SALE, 0) AS "M-2_Sale",
    NVL(C3.UNIT_OF_PHONE_SALE, 0) AS "M-3_Sale"
FROM
    CTE C
    LEFT JOIN CTE C1 ON ( C1.ORDER_MONTH = ADD_MONTHS(C.ORDER_MONTH, - 1) )
    LEFT JOIN CTE C2 ON ( C2.ORDER_MONTH = ADD_MONTHS(C.ORDER_MONTH, - 2) )
    LEFT JOIN CTE C3 ON ( C3.ORDER_MONTH = ADD_MONTHS(C.ORDER_MONTH, - 3) )
ORDER BY
    C.ORDER_MONTH DESC

ORDER_MONTH	M-1_Sale	M-2_Sale	M-3_Sale
201908	0	789	666
201906	666	765	0
201905	765	0	0
201904	0	0	0