;WITH 
TopHalf AS (
	SELECT TOP 50 PERCENT DataValue
	FROM DataSet
	ORDER BY DataValue ASC
)
,BottomHalf AS (
	SELECT TOP 50 PERCENT DataValue
	FROM DataSet
	ORDER BY DataValue DESC
)
,BottomOfTopHalf AS (
	SELECT TOP 1 DataValue
	FROM TopHalf
	ORDER BY DataValue DESC
)
,TopOfBottomHalf AS (
	SELECT TOP 1 DataValue
	FROM BottomHalf
	ORDER BY DataValue ASC
)
SELECT 
	Median = (BottomOfTopHalf.DataValue + TopOfBottomHalf.DataValue) / 2.0
FROM BottomOfTopHalf, TopOfBottomHalf