Use Atlantic;
GO
SELECT customer_ID
, (orders_qty * orders_sell_price) AS line_total
,	(SELECT CAST(AVG(orders_qty * orders_sell_price) AS DECIMAL(8,2))
	FROM Orders) AS avg_table_value
,	(orders_qty * orders_sell_price) -
		(SELECT CAST(AVG(orders_qty * orders_sell_price) AS DECIMAL(8,2))
		FROM Orders) AS variance
FROM  Orders
ORDER BY customer_id;