One or Both Foregin Keys

[Dennis Gearon <gearond@fireserve.net>]

I am working on a design where a location for something can have:

   Fully qualified address with even building names, room numbers, and 
booth numbers.
**-OR-**
   GPS location
**-OR-**
   Both

Other than triggers, is there a way to enforce this using tables and 
primary keys?

I imagine an address table, and then

either a some sort of intermediate table,  in a one to many 
relationship, GPS to Addresses,
or the correct columns in the address table.

Either way, I don't see a normal way to enforce EITHER or BOTH without a 
trigger.

Thanks in Advance.

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