Re: Entity R Diagrams/Case

From: Thomas Cox <tcox_at_qiclab.scn.rain.com>
Date: 24 Jun 93 20:31:27 GMT
Message-ID: <1993Jun24.203127.11745_at_qiclab.scn.rain.com>


jbestwic_at_uk.oracle.com writes:
> wayne_at_netcom.com (wayne t. watson) writes:

 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 _________________ /

Notice the ARC across the relationship lines. This denotes an exclusive or. The same relationship lines without the arc would be an inclusive or.

Yes, this will probably be correctly handled by the Oracle CASE tool. HOWEVER, *any* time you have an arc across the 'one and only one' side of your relationships, you're probably trying to implement a business rule as an arc, which is usually a Bad Idea.

Try encoding such rules as values in tables instead.

Also, *all* one-to-one relationship lines are suspect. You typically want to make them all one table.

(But the one-to-one relationship can be a very cool way to handle certain kinds of optional clusters of attributes. Unfortunately, Oracle CASE Dictionary v5.0 handles 1:1s completely wrong, with all sorts of stupid bouncing inheritance of primary keys.)

Cheers.

-- 
Thomas Cox      DoD #1776   '91 CB 750 Nighthawk   tcox_at_qiclab.scn.rain.com
    The Platinum Rule:  "Do Unto Others As They Want To Be Done Unto"
Received on Thu Jun 24 1993 - 22:31:27 CEST

Original text of this message