declare @validatedIDList nvarchar(max)

SELECT
    @validatedIDList = COALESCE(@validatedIDList + ', ', '') + CAST(si.SelectedID AS nvarchar(50))
FROM
    dbo.SelectedID si

DECLARE @validatedIDList NVARCHAR(max)

SELECT @validatedIDList = COALESCE(@validatedIDList + ', ', '') +
                          CAST(si.SelectedID AS NVARCHAR(50))
FROM   dbo.SelectedID si

SELECT internal_objects_alloc_page_count,
       internal_objects_dealloc_page_count,
       internal_objects_alloc_page_count - internal_objects_dealloc_page_count
FROM   sys.dm_db_task_space_usage
WHERE  session_id = @@SPID


SELECT DATALENGTH(@validatedIDList)

+-----------------------------------+-------------------------------------+-------+
| internal_objects_alloc_page_count | internal_objects_dealloc_page_count |       |
+-----------------------------------+-------------------------------------+-------+
|                           5571528 |                             5571424 |   104 |
+-----------------------------------+-------------------------------------+-------+

DECLARE @validatedIDList nvarchar(max)

SELECT @validatedIDList = STUFF((SELECT ', ' + COALESCE(CAST(si.SelectedID AS (nvarchar(50)), '')
FROM dbo.SelectedID si
FOR XML PATH('')), 1, 1, '')

DECLARE @SelectedID TABLE (IDs INT)
INSERT INTO @SelectedID VALUES
(1),(2),(null),(4),(5),(null),(7)


declare @validatedIDList nvarchar(max)


SET @validatedIDList =  STUFF((SELECT ', ' + CAST(si.IDs AS nvarchar(50))
                              FROM @SelectedID si
                              FOR XML PATH('')),1 ,2 ,'')

SELECT @validatedIDList


RESULT : 1, 2, 4, 5, 7