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From: JOG <jog@cs.nott.ac.uk>
Newsgroups: comp.databases.theory
Subject: Re: Examples of SQL anomalies?
Date: Mon, 7 Jul 2008 13:11:53 -0700 (PDT)
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On Jul 5, 1:55 pm, goanna <spamt...@crayne.org> wrote:
> Marshall <marshall.spi...@gmail.com> writes:
> > What can be meaningfully asked is determined by the schema.
>
> Close. What can be meaningfully asked is determined by the meaning
> of the schema.  In the case of nullable attributes, this must be
> specified, not guessed.
>
> > If the schema specifies that the weight attribute is nullable,
> > then the question of how much a shipment weighs in total
> > is a question that cannot be asked.
>
> If the schema specifies that the weight attribute is nullable,
> we need to know what, precisely, is the intended meaning of a
> null weight.

No we don't. It suffices to know that the question cannot be asked of
the RM, whatever it was supposed to mean.

> If may mean that the weight is unknown, or it may
> mean that the weight attribute is not applicable to this entity
> (e.g. the weight of an electronic book), or it may mean ...
> In a badly designed schema it may unfortunately be used with
> more than one meaning without permitting them to be distinguished.
>
> Nulls are often but not exclusively used to mean unknown value.
> Failure to distinguish between different uses of null is the
> basis of most of these, rather pointless, arguments.

Again, no. Theoretically nulls do not belong where a value should be.
This is not a comment on missing data, but on avoiding hemorrhaging
the relational model's underlying mathematics. We should be able to do
better than that in computer "science" after all.