Interviewvraag:Hoe krijg ik de aggregatie van de laatste 3 maanden op kolomniveau?

U moet eerst de gegevens van de maand optellen en vervolgens de LAG-functie gebruiken om de gegevens van de vorige maanden als volgt te krijgen:

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;

Uitvoer:

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<>fiddle-demo

Proost!!

-- Bijwerken --

Voor de vereiste die in de opmerkingen wordt vermeld, zal de volgende zoekopdracht ervoor werken.

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

Uitvoer:

db<>fiddle-demo van geüpdatet antwoord.

Proost!!