How to use CASE in MS SQL

case_usage.sql

WITH Orders (customerID, orderNum) AS
(
	SELECT CustomerID, COUNT(*) 'number of orders'
	FROM Sales.SalesOrderHeader
	GROUP BY CustomerID
)
SELECT customerID, 'Value' =
	CASE
		WHEN orderNum > (SELECT (AVG(CAST(orderNum AS FLOAT)))
						 FROM Orders
						) * 10 THEN 'Y'
		ELSE 'N' 
	END,
	orderNum
FROM Orders