Re: NULLs: theoretical problems?

On 9 aug, 23:25, "David Portas"
<REMOVE_BEFORE_REPLYING_dpor..._at_acm.org> wrote:
> "paul c" <toledobythe..._at_oohay.ac> wrote in message
>
> news:vnIui.42734$_d2.2251_at_pd7urf3no...
>
>
>
> > Hugo Kornelis wrote:
> >> On Wed, 08 Aug 2007 20:42:52 GMT, paul c wrote:
>
> >>> Hugo Kornelis wrote:
> >>>> ...
> >>>> The ANSI standard defines NULL as follows:
>
> >>>> "null value: A special value that is used to indicate the
> >>>> absence of any data value."
>
> >>>> Thus, the only concept collapsed into NULL is that of "no value here".
> >>>> ...
> > ...
> >> <nitpicking>
> >> The ANSI text doesn't define "the null value" (as they call it) as a
> >> value that conveys that the value is not _a_ value, but as a _special_
> >> value that conveys that there is no _data_ value.
> >> </nitpicking>
>
> >> Personally, I never call NULL a value, and I prefer to describe NULL as
> >> a "marker that indicates the absence of any value". But I couldn't
> >> really write that and still claim to be citing ANSI, eh?
> >> ...
>
> > I noticed in your blog you said that the table with a null-able birthday
> > column was in 2NF. If saying that is right, I suppose we must be careful
> > to regard functional dependencies as determining values sometimes and
> > non-values other times.
>
> But Hugo is not right. From the Alice Book, p163:
>
> "A relation I over U satisfies X -> Y, if for each pair s, t of tuples in I,
> s{X} = t{X} implies s{Y} = t{Y}."
>
> Since null = null is not true, no attribute with nulls satisfies any FD, not
> even the trivial one {A}->{A}!

A small warning. You are taking a predicate over a certain domain (namely relations as defined in the Alice book) and are now trying to apply to things outside that domain (relations with null values) for which it not necessarily makes sense. Of course, it wil still lead to interesting conversation and lots of armchair philosophizing, but probably will not produce anything meaningful.

• Jan Hidders