After extensive googling, I still can't find a definitive answer to this question. Some old articles/blog posts I've seen say not at all. Some say yes if the underling database supports it. Which is it? I asked on the nhusers group with no answer so far. Any help would be appreciated.
R – Does NHibernate HQL support the UNION ALL keyword
NHibernate does not support union. There is always one type in a result.
What you can do is a query to a base class or interface. NHibernate will perform a union to get all the values from all tables where the entities are mapped to.
There is also the
union-subclassmapping strategy, which also implies that there is a base class or interface.