DECLARE @foo TABLE (
	[val] CHAR(1)
	, [timestamp] DATETIME
	)

INSERT @foo
	      SELECT 'a', '2009-01-01T11:31:00'
UNION ALL SELECT 'b', '2009-01-01T12:00:00'
UNION ALL SELECT 'c', '2009-03-01T12:00:00'
UNION ALL SELECT 'd', '2009-03-01T17:00:00'
UNION ALL SELECT 'e', '2008-01-01T12:00:00'
UNION ALL SELECT 'f', '2008-01-01T12:20:12'
UNION ALL SELECT 'g', '2008-01-01T12:50:00'

-- Show all rows from @foo
SELECT * FROM @foo

-- Select rows that are within 30 mins of the row before (ordered by val ASC)
;WITH orders AS (
SELECT
	ROW_NUMBER() OVER(ORDER BY [val] ASC) AS [pos]
	, [val] AS [val]
	, [timeStamp] AS [timestamp]
FROM
	@foo
	)
SELECT
	a.*
FROM
	orders a

	JOIN orders b ON
		b.[pos] = a.[pos] + 1
		AND DATEDIFF(MINUTE, a.[timeStamp], b.[timeStamp]) BETWEEN 0 AND 31