Hi Gigi,
If you use an Entity as Source Entity of a Combo Box, it will sort the list according to the Order By Attribute of the Entity. If there's no Order By Attribute defined, it will sort based on Id.
However, if you specify the output of an Aggregate or SQL as Source Record List, the order that list has is maintained. Without seeing your code I cannot tell what exactly the problem is, but I would guess that you don't actually use the Aggregate output as Source Record List.