Entity R Diagrams/Case

I've been looking at an Addison Wesley book about the Oracle Case Tools. The author's name escapes me. He is British. Anyway, I have two questions regarding what I've read so far.

1. In an ER diagram, suppose you have something like Entity A

   ---

   
   | |
   | |- - - - ---------- B
   | |
   | |- - - - ---------- C
   

   ---

That is, Entity A has relationships with two other entities that are each of the 'may be' kind. But suppose these are exclusive OR and the appropriate symbol for this is placed across the symbols. Does this now mean either 'A may be related to B' or 'A may be related to C', so that one might expect either relationship, and possibly none? Assume The same relationships are 'must be'. Does the either/or imply always one or the other (but never none)? If in the first case ('may be') none is not a choice, then doesn't the exclusive OR imply the two relationships are then the same as 'must be'.

Ok, here is any easier one? Does the Oracle Case ER Diagram tool somehow generate the necessary tables from the diagram? I mean, at some point, in the ER diagram game, doesn't someone actually have to generate tables for each of the entitites (which I assume are really tables).

Well, here is another question. Who started all this ER Diagram business anyway?                         

-- 
Wayne T. Watson   wayne_at_netcom.com (415) 969-4233
Mountain View, CA