I am sorry the code should be:

Create FUNCTION GetCol4 (@col1 datatype, @col2 datatype, @col3 datatype)RETURNS varchar(5000)AS

BEGIN

DECLARE @Cols varchar(5000)

SET @Cols = ''



SELECT @Cols = 

CASE @Cols   

WHEN '' THEN col4   

ELSE @Cols + ', ' + col4

END

FROM sample

where col1 = @col1

and col2 = @col2

and col3 = @col3



RETURN(@Cols)