Re: Much ado about nothing
Date: Wed, 16 Oct 2002 18:05:07 +0200
Message-ID: <aok2nl$nftsp$1_at_ID-148886.news.dfncis.de>
David Cressey wrote:
> It seems to me that the discussion about SQL and NULLS is degenerating.
Well, it was intended as a criticism of SQL NULLs, so why do you think it is degenerating?
> At the outset, in any data representation scheme, it's generally useful to
> have a marker that indicates the absence of data, in a place where data is
> expected.
Except in the relational model, that is not a data representation scheme (physical) but a data model (logical).
> Mathematical relations have no need of the concept of NULLS, because tuples
> contain values, and only values.
> Result tables in SQL may need the concept of NULLS, where mathematical
> relations do not, because of the difference between SQL and mathematics.
Therefore, SQL is not relational.
> Many of the difficulties we experience with NULLS is due to the attempt to
> overload NULLS with explicit meaning. That is, we equate the failure to
> convey a message as being a message in and of itself. If we would simply
> agree that a NULL indicates the absence of data, and nothing more than that,
> we'd have a lot less trouble.
Agreed. But then SQL deals horribly wrong even if this simple, originally intended meaning of NULLs -- which isn't much useful, thus having prompted Codd to create his 4VL A- & I-mark scheme.
-- _ / \ Leandro Guimarães Faria Corsetti Dutra +41 (21) 216 15 93 \ / http://homepage.mac.com./leandrod/ fax +41 (21) 216 19 04 X http://tutoriald.sourceforge.net./ Orange Communications CH / \ Campanha fita ASCII, contra correio HTML +41 (21) 644 23 01Received on Wed Oct 16 2002 - 18:05:07 CEST