AutoCAD Civil 3D :: Show Inner Pipe Or Headwall Base Width In Table With Expression

I am trying to work out a way to set up my Structure Table so that I can show either the Inner Pipe Width OR Headwall Base Width to avoid having Question Marks showing in my Table? Is there a Function in the expressions that equates to a Non-Existent or Null Parameter. Ie.

IF({Inner Structure Width}=???,{Headwall Base Width})

Also, Is there a way to hide Structures from a Table, in particulat, Null Structures?

Currently using Civil 3D 2013 & 2014 SP1