Outer Join OTBI - Vacant Positions

Summary:

I am trying to produce an analytic that can show the number of "Vacant" positions. The analytic requires two subject areas and I believe I may need an outer join between the two.

Content (required):

At our organisation a Vacant position is;

Active

or;

It has either; never been assigned to a person or if it has, it has no current active assignments.

The analytic I first pulled together uses two subject areas

"Workforce Management - Position Real Time" and "Workforce Management - Worker Assignment Real Time"

The table view highlights the problems I am having; Were a position exists but there is no corresponding Assignment, the columns relating to assignment look blank and at first appear to be null. However, any formulae looking to these columns does not return a value. This makes me think that the are not null but simply nothing is being returned.