et non, ce que je veux c'est récupérer toutes les catégories d'une galerie x, et pour chaque catégorie, afficher les images. c'est pour ça que je fais ma 2eme requête, pour récupérer les catégories d'une galerie donnée.

/* 1 selection des galeries */	
	$sql_gal = "SELECT * FROM galerie WHERE Id='$gal'";
	$req_gal = mysql_query($sql_gal) or die('Erreur SQL'.mysql_error());
	$result_gal = mysql_fetch_array($req_gal);
	$nom_galerie = $result_gal['Name'];
	$chemin = $result_gal['Chemin'];

	echo 'debut_galerie';
	echo '<br/> id galerie = '.$result_gal['Id'];

/* 2 selection des categories */	
	
	$sql_cat = "SELECT * FROM categorie WHERE Id_Galerie='$gal' ORDER BY Name";
	$req_cat = mysql_query($sql_cat) or die('Erreur SQL'.mysql_error());
	
	while ($result_cat = mysql_fetch_array($req_cat)) {
		$cat_prec = '';
		$cat = $result_cat['Id'];
		

		echo '<br/>debut_categorie';
		
		echo '<br/> id categorie = '.$result_cat['Id'];


/* 3 selection des images */
		$sql = "SELECT * FROM image WHERE Id_Cat = '$cat' GROUP BY Id_Cat ORDER BY Annee DESC";
		$req = mysql_query($sql) or die(mysql_error());
			if (mysql_num_rows($req)>0) {
				echo '<br/> debut image';
				
					while ($result = mysql_fetch_array($req)) {
						$cat_prec = $cat;
						echo '<br> continue image';
						echo '<br/> id galerie = '.$gal;
						echo '<br/> id categorie = '.$result['Id_Cat'];
						echo '<br/> id image = '.$result['Id'];
					}
					
				echo '<br/> fin image';
				
			}

			echo '<br/> fin categorie';

	}

echo '<br/> fin galerie';

$flag = '';
while( $r = ... ){
  if( $flag != $r['flag'] ) {
    echo 'nouveau flag';
    $flag = $r['flag'];
  }
  echo $r['data'];
}

$sql = "SELECT * FROM image WHERE Id_Cat IN (SELECT categorie.Id FROM categorie WHERE categorie.Id_Galerie='$gal') ORDER BY Annee DESC GROUP BY Id_Cat";

	

$sql_gal = "SELECT * FROM galerie WHERE Id='$gal'";
	$req_gal = mysql_query($sql_gal) or die('Erreur SQL'.mysql_error());
	$nom_galerie = $result_gal['Name'];
	$chemin = $result_gal['Chemin'];
	
	$sql_cat = "SELECT * FROM categorie WHERE Id_Galerie='$gal';";
	$req_cat = mysql_query($sql_cat) or die('Erreur SQL'.mysql_error());

		
/* debut d'ecriture dans le xml*/



echo 'debut_galerie';


	
while ($result_gal = mysql_fetch_array($req_cat)) {


$nom_cat= $result_gal['Name'];

echo '<br/>debut_categorie';




$sql = "SELECT * FROM image WHERE Id_Cat IN (SELECT categorie.Id FROM categorie WHERE categorie.Id_Galerie='$gal') ORDER BY Annee DESC";

$req = mysql_query($sql) or die("mysql connect Failled on : selection de la categorie");


		if (mysql_num_rows($req)>0) {
				echo '<br/> debut image';
				
				while ($result = mysql_fetch_array($req)) {
	
				echo '<br> continue image';
				echo $result['Id_Cat'];
				echo '<br/>'.$result['Id'];
				
				}
				
                       echo '<br/> fin image';
		}

echo '<br/> fin categorie';

}
echo '<br/> fin galerie';