[Formulaires] Recherche avec différentes tables (indépendamment)

SELECT clients.nom
          Agence.Nom Agence
          specialites.DetailSpecialite
FROM  clients , Agences, specialites
Where clients.refNomAgence = Agences.refNomAgence
 And  clients.refSpecialites = specialites.detailsSpecialites
And  ref Nom Agence = 'Agence1';

<?php 

echo "<form method='post' action='ventes_rajout'>\n";
echo "<center>";
echo "<table class=\"green\">";
echo "<tr><td>Client</td>";
echo "<td>";

//recuperation de la liste des clients
$query      = "SELECT id_client, nom_client ";
$query     .= "FROM   clients   ";
$query     .= ";	";
$result = execute_query($query);

//creation select avec liste des clients
echo "<select id='id_client' name='id_client'>";
while($row = mysql_fetch_array($result)) {
	$id_client  = $row[0];
	$nom_client = $row[1];
	echo "	<option value='" . $id_client . "'>" . $nom_client . "</option>";
}//end while
echo "</select>";
echo "</td>";
echo "</tr>";

echo "<tr><td>      </td><td><input  type='submit'></td></tr>\n";
	
echo "</tr>";
echo "</td>";
echo "</table>";
echo "</form><br/>\n";

?>

if(isset($_REQUEST["id_client"   ])) {$id_client    = $_REQUEST["id_client" ];}
else {$id_client    = "";}