DECLARE @Min_Order INT

-- create temporary table to hold order id and filled status (N=No, Y=Yes, X=Can't be filled)
DROP TABLE #TempTable
CREATE TABLE #TempTable (OrderId INT, Filled CHAR(1))   

-- insert orders into temp table, set filled to 'N' 
INSERT INTO #TempTable
SELECT OrderId, 'N'
FROM ORDERS

-- for those orders where insufficient quantities available, set filled status to 'X'
UPDATE #TempTable
SET Filled = 'X'
FROM #TempTable T
JOIN OrderDetails OD
ON T.OrderId = OD.OrderId
JOIN Products P
ON OD.ProductId = P.ProductId
WHERE T.Filled = 'N'
AND OD.Quantity > (P.Inventory - P.Allocated)

-- loop through remaining unfilled orders
WHILE EXISTS (SELECT * FROM #TempTable WHERE Filled = 'N')
BEGIN
    -- get order id of first unfilled order
    SELECT @Min_Order = (SELECT MIN(OrderId) FROM #TempTable WHERE Filled = 'N')

    -- update allocated field of products table for order
    UPDATE Products
    SET Allocated = O.Quantity + P.Allocated
    FROM Products P
    JOIN OrderDetails O
    ON P.ProductId = O.ProductId
    WHERE O.OrderId = @Min_Order

    -- set filled status to 'Y'
    UPDATE #TempTable
    SET Filled = 'Y'
    WHERE OrderId = @Min_Order

    -- for those remaining orders where insufficient quantities available, set filled status to 'X'
    UPDATE #TempTable
    SET Filled = 'X'
    FROM #TempTable T
    JOIN OrderDetails OD
    ON T.OrderId = OD.OrderId
    JOIN Products P
    ON OD.ProductId = P.ProductId
    WHERE T.Filled = 'N'
    AND OD.Quantity > (P.Inventory - P.Allocated)

END

SELECT * FROM #TempTable

Select Distinct O.OrderID,
	Case sign(OD.Quantity - (Select sum(Quantity)
		from Orders O1
		inner join OrderDetails OD1 on O1.OrderID = OD1.OrderID
		where OD1.ProductID = P.ProductID and O.DatePlaced >= O1.DatePlaced
		and not exists  (Select O.OrderID
					from Orders O
					inner join OrderDetails OD on O.OrderID = OD.OrderID
					Inner join Products P on P.ProductID = OD.ProductID
					where Quantity > Inventory and O.OrderID = O1.OrderID
					group by O.OrderID )))
	when 1 then 'Fillable'
	when 0 then 'Fillable'
	else 'Not Fillable'
	end  as Status
from Orders O
inner join OrderDetails OD on O.OrderID = OD.OrderID
Inner join Products P on P.ProductID = OD.ProductID