Entities, Relationships, and Tables

From: David Cressey <david_at_dcressey.com>
Date: Tue, 24 Sep 2002 15:10:35 GMT
Message-ID: <Lf%j9.128$0I3.8044_at_petpeeve.ziplink.net>

In another thread, it was pointed out that a table that represents a relationship could be reconstrued
as representing an entity. As an example, an airline reservation system could treat a booking as a relationship between a passenger, a flight, and a date. Alternatively, we could construe an entity called a "reservation", and think of a table containing bookings as an entity table instead of a relationship table. I've sometimes seen that referred to as "reifying the relationship".

I'm wondering about the opposite construction. It seems to me that if you think of relationships as being of order 2, 3, 4 etc. (binary, ternary, quaternerary, etc.), all you have to do is extend the concept downward one, to "unary relationships", and you can now think of your entity tables as being "unary relationship tables". I'm not sure whether the concept of "unary relationship" is a useful one or not. I rather suspect not, or else I think I would have seen it somewhere.

However, there is a nice symmetry here. If all your tables represent relations, when viewed logically, then they also represent relationships, when viewed conceptually. Is this conceptual parsimony, or is it just small mindedness?

--
Regards,
    David Cressey
    www.dcressey.com

Received on Tue Sep 24 2002 - 17:10:35 CEST