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)