Re: object algebra
Date: Tue, 24 Feb 2004 21:51:03 GMT
Message-ID: <bHP_b.17521$%p4.2788_at_newssvr31.news.prodigy.com>
"Neo" <neo55592_at_hotmail.com> wrote in message
news:4b45d3ad.0402241256.660ae7e9_at_posting.google.com...
> I create a relation named T_Person.
> It has attibutes Name and EyeColor.
> The first tuple has attribute values John and blue.
> The second tuple has attribute values Mary and NULL. (She has no eyes).
>
> There now exists at least one tuple containing a NULL.
If Mary has no eyes, then the definition of a person as having eyes is flawed. However, if it's convenient to require some designation for eye color, NULL is a lousy way to do it. You should in this case have an EYE_COLOR type, and one of its values could be "Not Applicable" or "Unknown" (both of which have well-defined meanings, unlike NULL). Received on Tue Feb 24 2004 - 22:51:03 CET