DECLARE @Orders TABLE (OrdNo VARCHAR(20), [Date] Date, PartName VARCHAR(20), OrdQty INT);
    DECLARE @Balance TABLE (PartName VARCHAR(20), Balance INT);

    INSERT @Orders VALUES
        ('Заказ1', '1.2.18', 'Part1', 5),
        ('Заказ2', '1.1.18', 'Part1', 8),
        ('Заказ3', '1.5.18', 'Part1', 3);

    INSERT @Balance VALUES
        ('Part1', 20),
        ('Part2', 29);

    SELECT OrdNo, 
           [Date],
           Orders.PartName,
           OrdQty,
           Balance - IsNull(SUM(OrdQty) OVER (PARTITION BY Orders.PartName  ORDER BY [Date] ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING),0) AS Total31
      FROM @Orders AS Orders
INNER JOIN @Balance as Balance ON Orders.PartName = Balance.PartName