Domanda del colloquio:come ottenere l'aggregazione degli ultimi 3 mesi a livello di colonna?

Dovrai prima sommare i dati del mese e quindi utilizzare la funzione LAG per ottenere i dati dei mesi precedenti come segue:

SELECT
    ORDER_MONTH,
    LAG(UNIT_OF_PHONE_SALE, 1) OVER(
        ORDER BY
            ORDER_MONTH
    ) AS "M-1_Sale",
    LAG(UNIT_OF_PHONE_SALE, 2) OVER(
        ORDER BY
            ORDER_MONTH
    ) AS "M-2_Sale",
    LAG(UNIT_OF_PHONE_SALE, 3) OVER(
        ORDER BY
            ORDER_MONTH
    ) AS "M-3_Sale"
FROM
    (
        SELECT
            TO_CHAR(ORDERDATE, 'YYYYMM') AS ORDER_MONTH,
            SUM(UNIT_OF_PHONE_SALE) AS UNIT_OF_PHONE_SALE
        FROM
            DATAA
        GROUP BY
            TO_CHAR(ORDERDATE, 'YYYYMM')
    )
ORDER BY
    ORDER_MONTH DESC;

Risultato:

ORDER_   M-1_Sale   M-2_Sale   M-3_Sale
------ ---------- ---------- ----------
201908       3789        789        666
201907        789        666        765
201906        666        765
201905        765
201904

db<>demo violino

Ciao!!

-- Aggiorna --

Per il requisito menzionato nei commenti, la seguente query funzionerà.

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

Risultato:

db<>demo violino di risposta aggiornata.

Ciao!!