Orders come from companies with only one location, or from

companies with many locations – where invoices go to “HQ”.
Hi Barry
I’m thinking of creating a table BillTo that stores the address etc for the 
headquarters location, and including a foreign key (BillToID) in my Customer table. 
When an order comes in, if the BillToID is NULL (in Customer table) I would use the info 
in the Customer table. 
If it contained a value, I look up the info from the correct record in the BillTo table.
I also thought about setting up parent HQ / child LOCATIONS tables for customers as well. 
I thought this may complicate my (limited) ability to combine both tables to query.
90% of the time, the HQ or BillTo locations also belong in the Customer table. 
I would end up with half of my customers in the HQ table and half in the LOCATIONS table. 
Given your reply, it appears that you would prefer the HQ / LOCATIONS tables – am I correct? 
I want to do this structurally correct. You’re probably realizing that I’m NO database pro, 
and I certainly appreciate your time and input. I hope my ramblings make sense.
Thank You Very Much Barry!
Paul

A.The Approach
There are three Steps in our Approach :-
1.	Identify the ‘Things  of Interest’ in the scope of the Data Model
          We aim to clarify any ambiguities at this stage.
2.	Determine how they are related.
3.	Establish what else we know about them.
B. The ‘Things  of Interest’  (in alphabetical order) :-
1.	Addresses
2.	Address Types
3.	Companies
4.	Company Type can be either a Wholesaler, a Contractor, an Engineer, 
        an Architect, School, Retail Store, 
5.	Contacts
6.	Contractors
7.	Customers
8.	Invoices
9.	Locations
10.	Orders
11.	Quotes
12.	Wholesalers
C. How these  ‘Things’  are related :-
1.	A Contact works for one Company
2.	Correspondence is sent to Contacts
3.	A Customer can be either a Company or an Individual (a Contractor).
4.	A Customer is a Company or Individual that has purchased or wants to purchase something.
5.	Invoices are sent to Companies  (Customers ?)
6.	Quotes are sent to Contacts
Questions to be resolved :-
1.	Should 5 say ‘Invoices are sent to Customers’.
D. What else do we know about them ? :-
1.	Address Types can be either Billing, Mailing (for example PO Box), Physical or Shipping
2.	A Contact has a first, middle and last name
3.	A Company has a name