Re: What are the differences between the terms, CANDIDATE KEY, PRIMARY KEY, SUPER KEY, COMPOSITE KEY?
From: Marshall Spight <marshall.spight_at_gmail.com>
Date: 24 Feb 2006 10:16:34 -0800
Message-ID: <1140804994.799874.324810_at_t39g2000cwt.googlegroups.com>
> There is no formal distinction between them. Therefore, let's speak of
> unique key or just a key. Formally, a key is a functional dependence
> from a set of attributes to the whole relation header.
Date: 24 Feb 2006 10:16:34 -0800
Message-ID: <1140804994.799874.324810_at_t39g2000cwt.googlegroups.com>
Mikito Harakiri wrote:
>
> There is no formal distinction between them. Therefore, let's speak of
> unique key or just a key. Formally, a key is a functional dependence
> from a set of attributes to the whole relation header.
This is nice and simple, but it slightly bothers me that in most cases, this describes an FD that has attributes on both the left and right side of the arrow. (Not that there is anything wrong with that, but perhaps there is some value to excluding the trivial depedencies?)
What would you say to:
A functional dependency X -> Y, where X and Y are disjoint sets of attributes, is a key if X union Y = relation header.
Is it always possible to uniquely normalize the set of functional dependencies such that there are no trivial dependencies?
Marshall Received on Fri Feb 24 2006 - 19:16:34 CET