Re: Recursive (Unary) Relatinoships

From: Jan Hidders <hidders_at_REMOVE.THIS.win.tue.nl>
Date: 12 Nov 2000 17:20:50 GMT
Message-ID: <8umjhi$49l$1_at_news.tue.nl>


gkiourtas michail wrote:
> Can someone present the following meanings in an easy to understand way?
>
> What is the usefullness of these meanings in database design?
> a) Symmetric Recursive Relationship

A binary relationship is recursive if it holds between entities of the same class. E.g. the friend-of relationship holds between two persons. Such a relation is symmetric if it holds that the relationship holds between p1 and p2 then it also holds between p2 and p1. In this example this would mean that if p1 is a fried of p2 then p2 is also a friend of p1. The usefulness of this concept is that it defines a database constraint that should be guarded by the database.

By the way. A unary recursive relationship is not really a meaningful concept in database theory.

> b) Asymmetric Recursive Relationship

A relation is asymmetric if it holds that if the relationship holds between p1 and p2 then it does *not* hold between p2 and p1 (unless p1 and p2 are one and the same).

> c) Recursive Relationship as an Entity.

Any relationship, recursive or not, can be promoted to an Entity.

-- 
Kind regards,

    Jan Hidders
Received on Sun Nov 12 2000 - 18:20:50 CET

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