Nette\Database\Selection select sum nad vice sloupecky

Ahoj, chtel bych s pouziti nette database zavolat select sum nad vice sloupecky.
Priklad: select sum(num) as num, sum(num1) as num1 from…

Kdyz si tu sumu poskladam do stringu a predam funkci Nette\Database\Selection::select(),
tak to na me vyplivne:

SQLSTATE[42601] coz je syntax error::

Jak spravne poskladat ten select? Doposavad mam:

$sum = "sum(num) as num, sum(num1) as num1";
$this->getView()
     ->where(..)
     ->group(..)
     ->select($sum)
     ->fetchAll();

Diky