UPDATE a
	SET StDev_OH_30Day=(SELECT STDEV(b.OHdiff) 
							FROM myTable b 
							WHERE b.IDNumber= a.IDNumber
								AND b.MarketDate < a.MarketDate 
								AND b.MarketDate > a.MarketDate-31 )
		, StDev_OL_30Day=(SELECT STDEV(b.OLdiff) 
							FROM myTable b 
							WHERE b.IDNumber = a.IDNumber 
								AND b.MarketDate < a.MarketDate 
								AND b.MarketDate > a.MarketDate-31 )
	FROM myTable a
	WHERE 
		a.MarketDate BETWEEN @lastUpdate+.1 AND @updateThrough
		AND a.IDNumber BETWEEN @minIDNumber AND @maxIDNumber

	UPDATE a
	SET StDev_OH_30Day=T.StDev_OHdiff
		, StDev_OL_30Day=T.StDev_OLdiff
	FROM myTable, 
		(SELECT STDEV(b.OHdiff) AS StDev_OHdiff
				, STDEV(b.OLdiff) AS StDev_OLdiff
			FROM myTable b
			WHERE b.IDNumber = a.IDNumber 
				AND b.MarketDate < a.MarketDate 
				AND b.MarketDate > a.MarketDate-31 ) AS T
	WHERE 
		a.MarketDate BETWEEN @lastUpdate+.1 AND @updateThrough
		AND a.IDNumber  BETWEEN @minIDNumber  AND @maxIDNumber 

;
WITH get_stddev AS
     ( SELECT  a.idnumber               ,
              a.marketdate              ,
              STDEV(b.OHdiff) stddev_oh ,
              STDEV(b.OLdiff) stddev_ol
            
     FROM     myTable b
              JOIN myTable a
              ON
                       (
                                b.IDNumber   = a.IDNumber
                            AND b.MarketDate < a.MarketDate
                            AND b.MarketDate > a.MarketDate-31
                       )
     WHERE    a.MarketDate BETWEEN @lastUpdate + .1 AND @updateThrough
          AND a.IDNumber BETWEEN @minIDNumber AND @maxIDNumber
     GROUP BY a.idnumber,
              a.marketdate
     )        ----- end cte
UPDATE c
SET    StDev_OH_30Day = stddev_oh ,
       StDev_OL_30Day = stddev_ol
FROM   mytable c
       JOIN get_stddev gs
       ON
              (
                     gs.IDNumber  = c.IDNumber
                 AND c.marketdate = gs.marketdate )