Nějak takhle:

$sql = "SELECT DISTINCT id_uzivatele FROM uzivatel ORDER BY id_uzivatele";
$vysledek = mysql_query( $sql ) or die( mysql_error() );
	if ( mysql_num_rows( $vysledek ) > 0 )
	{
	 while ( $radek_s = mysql_fetch_array( $vysledek ) )
	 {
          //$code - musí se v cyklu naplnit
          $code = "sd58_fgsdfg";
          $sql2 = "UPDATE `uzivatel` SET code = '". $code. "' WHERE `id_uzivatele` = '". $radek_s['id_uzivatele']. "' LIMIT 1";
          mysql_query($sql2) or die("Neplatný dotaz: ". mysql_error());
         }
	}