In other words, I have a conversion table of all different units. If
there is no conversion between 2 units (such as volume and area) then
the sum returns null.


Shouldn't that return NULL IOW unknown ?

Karsten

I am not familiar with returning unknown. I know that according to the spec an aggregate should only return null if all of its values of the aggregate are null. If there is a way to return unknown in a non-NULL way, then that would be preferred.