DECLARE	@Sequence INT = 2;

DECLARE	@Sample TABLE
	(
		ID INT NOT NULL,
		x FLOAT NOT NULL
	);

INSERT	@Sample
	(
		ID,
		x
	)
VALUES	(0, 11),
	(2, 12),
	(4, 21),
	(5, 22),
	(7, 41),
	(8, 61),
	(9, 91);

WITH cteMedian
AS (
	SELECT	(ROW_NUMBER() OVER (ORDER BY ID) - 1) / @Sequence AS seq,
		ID,
		x
	FROM	@Sample
)
SELECT		seq,
		MIN(ID) AS minID,
		MAX(ID) AS maxID,
		AVG(x) AS Median
INTO		#Median
FROM		(
			SELECT	seq,
				ID,
				x,
				2 * ROW_NUMBER() OVER (PARTITION BY seq ORDER BY x) - COUNT(*) OVER (PARTITION BY seq) AS y
			FROM	cteMedian
		) AS d
WHERE		y BETWEEN 0 AND 2
GROUP BY	seq;

WITH cteDeviation
AS (
	SELECT	(ROW_NUMBER() OVER (ORDER BY ID) - 1) / @Sequence AS seq,
		ID,
		x
	FROM	@Sample
)
SELECT		seq,
		MIN(minID) AS minID,
		MAX(maxID) AS maxID,
		MIN(Median) AS Median,
		AVG(ABS(x - Median)) AS Deviation
FROM		(
			SELECT		d.seq,
					d.ID,
					d.x,
					2 * ROW_NUMBER() OVER (PARTITION BY d.seq ORDER BY ABS(d.x - m.Median)) - COUNT(*) OVER (PARTITION BY d.seq) AS y,
					m.Median,
					m.minID,
					m.maxID
			FROM		cteDeviation AS d
			INNER JOIN	#Median AS m ON m.seq = d.seq
		) AS d
WHERE		y BETWEEN 0 AND 2
GROUP BY	seq
ORDER BY	seq;

DROP TABLE	#Median;

quote:

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Originally posted by SwePeso

A set-based approach[code]DECLARE @Sequence INT = 2;

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