SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

WITH TestTable AS (
    SELECT 201811 AS Month, 333 AS Branch, 2 AS Supplier, 3122 AS Article, 4 AS SalesQty, 11 AS StockQty UNION ALL
    SELECT 201811, 345, 1, 1234, 2, 10 UNION ALL
    SELECT 201811, 345, 1, 4321, 3, 11 UNION ALL
    SELECT 201812, 333, 2, 3122, 2, 4  UNION ALL
    SELECT 201812, 345, 1, 1234, 3, 12 UNION ALL
    SELECT 201812, 345, 1, 4321, 4, 5  UNION ALL
    SELECT 201901, 333, 2, 3122, 1, 8  UNION ALL
    SELECT 201901, 345, 1, 1234, 6, 9  UNION ALL
    SELECT 201901, 345, 1, 4321, 2, 8  UNION ALL
    SELECT 201902, 333, 2, 3122, 7, NULL UNION ALL
    SELECT 201902, 345, 1, 1234, 4, 13 UNION ALL
    SELECT 201902, 345, 1, 4321, 1, 10
),
cte AS (
    SELECT t.*, ROW_NUMBER() OVER (PARTITION BY Branch, Supplier, Article
                                   ORDER BY Month DESC) rn,
        SUM(SalesQty) OVER (PARTITION BY Branch, Supplier, Article) SumSalesQty
    FROM TestTable t
)

SELECT
    Month,
    Branch,
    Supplier,
    Article,
    SumSalesQty,
    StockQty
FROM cte
WHERE rn = 1;

Month	Branch	Supplier	Article	SumSalesQty	StockQty
201902	333	2	3122	14	null
201902	345	1	1234	15	13
201902	345	1	4321	10	10