CASE statement in UPDATE query

UPDATE a
SET a.Date_Added = (CASE 
                    WHEN a.Date_Requested > b.ADmodifyTimeStamp THEN getdate() 
                    ELSE b.ADmodifyTimeStamp END)
FROM [dbo].[tableA] a INNER JOIN [dbo].[tableA] b 
     ON a.[col1] = b.[col1] AND a.[col2] = b.[col2]
WHERE a.[col3] IS NULL