始终返回0而不是计算的QTY的函数

Question

我在SQL中使用一种CAST函数，一种有效，而一种无效。这是我所拥有的：

SELECT
CUST_ORDER_LINE.ORDER_QTY AS Order_QTY
, CUST_ORDER_LINE.ORDER_QTY
, CUST_ORDER_LINE.UNIT_PRICE
, CAST(((CUST_ORDER_LINE.ORDER_QTY - CUST_ORDER_LINE.ORDER_QTY) * CUST_ORDER_LINE.UNIT_PRICE) AS decimal(15, 2)) AS Total_Order_Dollars -- Doesnt Work
, CAST((CUST_ORDER_LINE.ORDER_QTY - CUST_ORDER_LINE.ORDER_QTY) AS decimal(15, 2)) AS Quantity_To_Ship -- Works perfectly.

在第一个“ CAST”函数中总是返回0而不是计算的QTY是什么意思？

谢谢