Re: Does Codd's view of a relational database differ from that ofDate& Darwin? [M.Gittens]

Alexandr Savinov wrote:
> The situation with NULLs clearly demonstrates some serious problems of
> the relational model. I personally do not see any problem with NULLs but
> I am not thinking in terms of relational model. I've read this paper
>
> http://www.hughdarwen.freeola.com/TheThirdManifesto.web/Missing-info-without-nulls.pdf
>
>
> and I am really surprised - it provides completely unacceptable
> "solution" - it compomises the whole relational model IMO. NULL values
> have an absolutely concrete meaning and it does not matter how we call
> them after that. And this meaning, the things with such a semantics, is
> the basis of the model. This semantics can be expressed as "the absence"
> of thing. Any model starts from empty state before it can be populated
> with other kind of things and this initial state is strongly associated
> with the semantics of absence. If something disappears, if something is
> deleted then acutally we get null.

But the absence of something is done at the row level, not at the attribute level. If a proposition is true, it gets represented by a row in a table of the database. If not, it is absent from the database.

Here's some discussion of a very concrete example of a NULL problem in SQL: http://www.firstsql.com/iexist2.htm

The author says that his RDBMS solves this problem though, and that is just a problem with SQL and NULLs, not with the relational model and NULLs.

Are there any rebuttals of this claim that anyone has? Any concrete examples of logical inconsistencies caused by NULLs?

I guess the other claim against NULLs is that they make things more complex and less intuitive, which is a more subjective point. You could argue that Darwen's method of avoiding NULLs is more complex that using NULLs in the first place.

Paul. Received on Mon Jun 06 2005 - 15:48:35 CEST