Logical Column Mapping to Multiple Physical?

Question

I am "learning" OBIEE still and a consultant today showed me something that totally blew my mind
In the OBIEE Admin tool (in verson 10) there are 3 layers displayed. The physical, business management and presentation layer.
I was under the impression that columns would be "mapped" one to one from each layer
The consultant proved me wrong and showed me that in fact, when making a logical column that you can have it "map" to two physical column names
Since every minute with the consultant is $$$ I tried to google around to find out why and how is this so but havent been able to find anything
Could someone explain to me why this would be done?
For example I have the logical column named "Home Faculty" and it is mapped to two physical columns "Faculty Short Desc" & "Main Faculty"
How would OBIEE know which one to use when?
Or does it simply "merge" the data between the two columns into one?

Answer 1

In this case equi join.
When you are using OBIEE try to see wrt fact table with Table 1 and 2 and follow as I suggest earlier.
When you use Fact,Table 1 and 2
Physical Layer Joins would be like:
Fact join with Table 1
Fact join with Table 2
In BMM I can make 3 tables
Like
Fact
dim 1
dim 2
Other case would be
Physical Layer Joins would be like:
Fact join with Table 1
Fact join with Table 2
Table 1's A1,A2 equals Table 2's A1,A2
In BMM I can make 2 tables
Like
Fact
dim sourcing 1 and 2
a) 1 mapping with 2
b) dim with LTS 1 and 2
Hope this helps
If I made it more confuse ;) send me email with US mobile# will call you back
If helps mark