Get items order by a date property

Hi,

I am using following query to get items, it gets all the items if I don't use 'order by'.

BLOCKED SCRIPT

var Items_ = inn.newItem("Formulas","get");
     Items_.setAttribute("where","[Formulas].state = 'Approved' order by [Formulas].approved_date desc ");
     var results = Items.apply();

This is not getting the items. How to use order by in the above query?

Thank you.