Why doesn't this work? I'm trying to first compute NetAmount, then use it in the next column:

SELECT	t.decTrade_Ext_Price + t.decTrade_Shipping_Charge + t.decTrade_Other_Charge AS NetAmount,
		(t.decTrade_Product_Balance / t.decTrade_Qty) * NetAmount AS NotShip
	FROM Trade t
	JOIN Trading_Partner tp ON tp.iTrading_Partner_ID = t.iTrading_Partner_ID
	JOIN Product p ON p.sProduct_CD = t.sProduct_CD

However, this does work with the computation included in the column

SELECT	(t.decTrade_Product_Balance / t.decTrade_Qty) * (t.decTrade_Ext_Price + t.decTrade_Shipping_Charge + t.decTrade_Other_Charge) AS NotShip
	FROM Trade t
	JOIN Trading_Partner tp ON tp.iTrading_Partner_ID = t.iTrading_Partner_ID
	JOIN Product p ON p.sProduct_CD = t.sProduct_CD