关于ISNULL的问题

ALTER Proc OrderRefund_Confirm

  @OrderRefund_ID varchar(50),

  @operate varchar(50)

as

--========================

--订单退货确认

--========================

--eg :exec OrderRefund_Confirm '20120221000002', 1





declare @OrdersC_ID varchar(50),@Return_Qty  numeric(10,5),

        @Product_Code varchar(50) ,@Product_Name varchar(50),@Descript varchar(50), 

	@Project  varchar(50) , @Camera  varchar(50) ,@Logo  varchar(50) ,

        @Color  varchar(50) ,@Unit varchar(50),

        @Amount NUMERIC(10,5), @ErrorMsg varchar(2000) 



	

	if @operate=1 

	begin 

	    SELECT @OrdersC_ID=OrdersC_ID ,@Return_Qty=Return_Qty ,

	           @Product_Code=Product_Code , @Product_Name=Product_Name ,

	           @Descript=Descript , @Project=Project , @Camera=Camera , @Logo=Logo ,

		   @Color=Color , @Unit=Unit

		FROM OrderRefund 

	        WHERE OrderRefund_ID=@OrderRefund_ID



 

		if not exists(select * from WasteProducts

                    	where Product_Code=@Product_Code and Product_Name=@Product_Name 

                        and Descript=@Descript  and  Project=@Project 

                        and Camera=@Camera and Logo=@Logo

			and Color=@Color and Unit=@Unit)

		begin

			insert into WasteProducts(Product_Code,Product_Name,Descript,Project,Camera,Logo,Color,Unit,Amount)

		                           values(@Product_Code,@Product_Name,@Descript,@Project,@Camera,@Logo,@Color,@Unit,@Return_Qty)

		end

		else

		BEGIN   	

			--更新库存(预扣数量增加,库存数量增加)

	        	UPDATE WasteProducts SET Amount=isnull(Amount,0)+@Return_Qty 

	    	            WHERE Product_Code=@Product_Code and Product_Name=@Product_Name and Descript=@Descript 

		                    and Project=@Project and Camera=@Camera and Logo=@Logo 

							and Color=@Color and Unit=@Unit  

	    	

		       --更新订单(退货数量增加)   

		       UPDATE OrdersC SET Return_Qty=isnull(Return_Qty,0)+@Return_Qty WHERE OrdersC_ID=@OrdersC_ID

	         END

	end