Re: No exceptions?
From: Jonathan Leffler <jleffler_at_earthlink.net>
Date: Fri, 30 Jun 2006 06:35:39 GMT
Message-ID: <%C3pg.319$PE1.172_at_newsread2.news.pas.earthlink.net>
Date: Fri, 30 Jun 2006 06:35:39 GMT
Message-ID: <%C3pg.319$PE1.172_at_newsread2.news.pas.earthlink.net>
J M Davitt wrote:
> Maybe a fine point, but...
>
> All the attributes in a relation comprise, at least, a superkey. The
> set of attributes that qualify as a candidate key must hold unique
> values and no subset of those attributes must hold unique values. The
> only relations that could have empty candidate keys are those with
> empty headings, right? (I remember puzzling this when I first
> encountered DEE and DUM.) If the heading is not empty, must a
> non-empty subset of those attributes be declared as a key?
-- Jonathan Leffler #include <disclaimer.h> Email: jleffler_at_earthlink.net, jleffler_at_us.ibm.com Guardian of DBD::Informix v2005.02 -- http://dbi.perl.org/Received on Fri Jun 30 2006 - 08:35:39 CEST