我已经写了一个存储过程来计算净产品金额的基础上，一些折扣方案在数据库中可用。。。。存储过程编译成功，但在运行时出现错误1172（42000）：结果由多行组成
我的代码如下，

delimiter //
    CREATE PROCEDURE calculateNetItemAmount(IN productId INT,IN quantity INT, OUT netItemAmount DOUBLE)
    BEGIN
        DECLARE discountPer DOUBLE DEFAULT 0;

        SELECT `SellingUnitPrice` into netItemAmount 
        FROM `TBL_PRODUCT_MASTER` WHERE `Id` = productId;
        SET netItemAmount = quantity * netItemAmount ;

        SELECT discDetail.`DiscountPercentage` into discountPer 
        FROM `TBL_DISCOUNT_DETAIL` AS discDetail , `TBL_DISCOUNT` AS disc, TBL_PRODUCT_DISCOUNT AS prodDisc 
        WHERE disc.`DiscountStartDate` < NOW() 
        AND disc.`DiscountEndDate` > NOW() AND disc.`IsEnabled` = 1 
        AND disc.`SchemeType` = 'Quantity Discount' 
        AND discDetail.DiscountId = disc.Id AND prodDisc.productId = productId 
        AND prodDisc.`IsEnabled` = 1 ;

        IF discountPer IS NOT NULL THEN 
           SET netItemAmount = netItemAmount * (1 - discountPer * 0.01)        
       END IF;

        SELECT discDetail.`DiscountPercentage` into discountPer 
        FROM `TBL_DISCOUNT_DETAIL` AS discDetail , `TBL_DISCOUNT` AS disc, TBL_PRODUCT_DISCOUNT AS prodDisc 
        WHERE disc.`DiscountStartDate` < NOW() AND disc.`DiscountEndDate` > NOW() 
        AND disc.`IsEnabled` = 1 AND disc.`SchemeType` = 'Volume Discount' 
        AND discDetail.DiscountId = disc.Id AND prodDisc.productId = productId 
        AND prodDisc.`IsEnabled` = 1 ;

        IF discountPer IS NOT NULL THEN 
           SET netItemAmount = netItemAmount * (1 - discountPer * 0.01)
        END IF;

    END//
    delimiter ;

输出：

mysql> CALL calculateNetItemAmount(1, 2, @RES);
ERROR 1172 (42000): Result consisted of more than one row

请帮帮我…先谢谢你。。