Maximum value from the multiple columns for each unique column (primary key) in a table

Here the main purpose is to get the maximum value from date1,date2,date3,date4 for each of the product, where ProdId is a unique column (Primary key column)

DECLARE @table TABLE

(

      ProdId Int,

      date1 datetime,

      date2 datetime,

      date3 datetime,

      date4 datetime

)

INSERT INTO @table VALUES (1,'2014-09-20 08:14:57.647','2014-09-30 08:15:16.657','2014-09-28 08:15:28.067','2014-10-01 08:15:43.290')

INSERT INTO @table VALUES (2,'2014-09-21 08:14:57.647','2014-09-22 08:15:16.657','2014-09-23 08:15:28.067','2014-09-24 08:15:43.290')

SELECT * FROM @table

--- Method 1 ----

SELECT ProdId,Maxdate

FROM @table

CROSS APPLY (SELECT MAX(d) Maxdate FROM (VALUES (Date1), (Date2), (Date3), (Date4)) AS a(d)) md

--- Method 2 ----

SELECT ProdId,MAX(Maxdate)Maxdate

FROM @table

UNPIVOT

(

      Maxdate FOR E IN (Date1,Date2,Date3,Date4)

)AS Unpvt

GROUP BY ProdId

--- Method 3 ----

SELECT ProdId,MAX(dt)Maxdate

FROM

(

      SELECT ProdId,MAX(date1)dt FROM @table GROUP BY ProdId

      UNION

      SELECT ProdId,MAX(date2) FROM @table GROUP BY ProdId

      UNION

      SELECT ProdId,MAX(date3) FROM @table GROUP BY ProdId

      UNION

      SELECT ProdId,MAX(date4) FROM @table GROUP BY ProdId

)x

GROUP BY ProdId