Php mysql GROUP_CONCAT et GROUP BY

<?php
$sql = 'SELECT GROUP_CONCAT(" ", Contenance) AS ContenanceX FROM tb_produit WHERE Ensemble = "'.Valeur.'" ';
$req = mysql_query($sql) or die('Erreur SQL !<br>'.$sql.'<br>'.mysql_error());
while($data = mysql_fetch_assoc($req)) { ?>
<?php echo $data['ContenanceX']; ?>
<?php }; ?>

GROUP BY

Contenance
1L
1L
1L
4L
4L
4L

Formats disponibles : 1L, 1L, 1L, 4L, 4L, 4L