Problema con mysql_num_rows()

[piero.mac]

$query = "SELECT count(*) 
          FROM messaggio 
          where cod_associato = '".$cod."'"; 
$query_count = mysql_query($query) or die (mysql_error());

$count = mysql_fetch_row($query_count);
                                                                                               
if ($count[0] == 0) {
     echo 'ciao';
}