试图计算空值

Question

我当前正在尝试编写一个存储过程，该过程将显示所选的产品类型并计算所述产品的数量减去已租出但没有退货日期以显示多少的产品数量上述产品中的一部分目前可供出租。

到目前为止，仅尝试了“为空，为空，不为空”。我的一部分也认为我的COUNT个变量有误。

——-

ALTER PROCEDURE AvailableProducts(@ItemType VARCHAR(50)= NULL)

AS 

BEGIN

    DECLARE @AvailableCount INT

    IF @ItemType IS NOT NULL

        BEGIN

            SELECT @AvailableCount=COUNT(SerialNumber.ProductID)-COUNT(Rental.SerialNumber) 
            FROM PRODUCT
            INNER JOIN SerialNumber
                ON Product.ProdID=SerialNumber.ProductID
            INNER JOIN Rental 
                ON SerialNumber.SerialNumber=Rental.SerialNumber
            WHERE ProdDescription=@ItemType AND @AvailableCount>0
            SELECT COUNT(*) AS 'Number Available', RENTAL.StoreID,ProdDescription, ProdName,ProdSize, ProdFlex, ProdTerrain, ProdAbilityLevel,ProdShape,ProdRocker
            FROM PRODUCT
            INNER JOIN SerialNumber
                ON Product.ProdID=SerialNumber.ProductID
            INNER JOIN Rental 
                ON SerialNumber.SerialNumber=Rental.SerialNumber
            WHERE ProdDescription=@ItemType
            GROUP BY RENTAL.StoreID, ProdDescription, ProdName,ProdSize, ProdFlex, ProdTerrain, ProdAbilityLevel,ProdShape,ProdRocker
            ORDER BY 2,1
            RETURN (0);
        END
    ELSE
        BEGIN
            PRINT 'Please Enter one of the following Products:
             "Snowboard"
             "Skies"
             "Splitboard"
             "Boots"
             "Goggles"
             "Helmet"
 to Show Products that are available to rent.'
        RETURN (1);
        END
    END;
;