$sql =  "DELETE " .
                "   r.*," .
                "   a.* " .
                "FROM " .
                "   $db_rechnung AS r, " .
                "   $db_artikel AS a " .
                "WHERE " .
                "   r.id = '".$this->rechnungsid."' " .
                "OR " .
                "   a.belongsto = r.id";