If Then Else in Infopath (Xpath): how to ?

I've a module in InfoPath / Sharepoint, a query field, and some fields. I want to perform in formula a simple :

If (Condition) then
   True Condition
Else 
   False Condition

In particular I want to write this formula:

If (FieldA = "") then Get FieldB else Get FieldA

How to do in InfoPath formula ?

Thanks