$sql = "SELECT s.id AS sid, s.sektor, alts.id AS altsid, alts.alt_sektor
FROM sektorler AS s, alt_sektorler AS alts
WHERE s.id=alts.sektor_id ORDER BY s.sektor ASC";

$q = mysql_query($sql) or die(mysql_error());

  while ($a = mysql_fetch_array($q)) {
  
     if ($aktif_sektor == "") {
     $aktif_sektor = $a[sektor];
     echo '<b><a href="sektor.php?id='.$a[sid].'">'.$a[sektor].'</a></b><br>';
     }
     
     if ($aktif_sektor == $a[sektor]) {
     echo '<a href="alt_sektor.php?id='.$a[altsid].'">'.$a[alt_sektor].'</a> &nbsp; ';
     }
     
     else {
      $aktif_sektor = $a[sektor];
     echo '<br><br><b><a href="sektor.php?id='.$a[sid].'">'.$a[sektor].'</a></b><br>
     <a href="alt_sektor.php?id='.$a[altsid].'">'.$a[alt_sektor].'</a> &nbsp; ';
     }
     
   }