union all i group by Opcje

[masterbx2]

mam 2 zapytania do bazy ktore lacze UNION ALL. z obydwu dostaje wyniki z tej samej kolumny. w tym przypadku user_id. chialbym teraz zkozystac z group by user_id aby wyniki sie nie powtarzaly. Jezeli jednak dodaje do zapytania SELECT x FROm ($zapytanie) GROUP BY x wyskakuje blad. jak to rozwiazac?

[PHP] pobierz, plaintext 
 
$both = "
     	SELECT user_id FROM
		 (
 
		SELECT user_id FROM pm AS a2 INNER JOIN users AS b2 ON a2.pm_from = b2.user_id 
		WHERE a2.pm_to = '$user_id' AND a2.pm_del_to = '0'
		UNION ALL
		SELECT user_id FROM pm AS a INNER JOIN users AS b ON a.pm_to = b.user_id 
		WHERE a.pm_from = '$user_id' AND a.pm_del_from = '0'
		) GROUP BY user_id";		
 
		$wynik = mysql_query($both);
 
 
		while($row = mysql_fetch_array($wynik))
		{
			echo$row['user_id'];	
		}
 
[PHP] pobierz, plaintext