Re: So what's null then if it's not nothing?

vc wrote:
> JOG wrote:
> > vc wrote:
> > > Jon Heggland wrote:
> > > > In article <1134052742.347560.142840_at_o13g2000cwo.googlegroups.com>,
> > > > boston103_at_hotmail.com says...
> > > > >
> > > > > > I don't think a "regular" unknown/missing SQL NULL for a 2VL boolean
> > > > > > domain should be regarded a truth value. That would be inconsistent with
> > > > > > how NULL works in other domains.
> > > > >
> > > > > Then the logic ceases to be such if its truth values set include a
> > > > > value for which the equality predicate evaluates to anything other than
> > > > > TRUE or FALSE as I said elsewere.
> > > >
> > > > It does *not* include such a value. NULL is not a truth value any more
> > > > than it is a number or a string.
> > >
> > > I am missing something. If you store/use NULL as a logical value, haw
> > > can it *not* belong to the logical vaue domain with its logical
> > > operations? Sorry, but that does not make sense.
> > [snip]
> >
> > But Null can never _be_ a logical value: it is by definition an
> > indicator of the very absence of a logical value. In addition, as a
> > logical value how could it possibly exist?

>

That you cannot mathematically incorporate the null concept into a logical system in the way that has been proposed (i.e. 3VL, which obviously can be effective with more valid domains). In a boolean column for example you could not place true, false and Null in a mathematically consistent system. (i.e. it necessarily requires human's to interpret the inconsistency in the practical world)

>

The argument that would incorportate Null != Null into the logical arrangement - this would mean that the equality relation in the domain would be non-reflexive, a nonsense in a logic system. This should be the stopping point for that train of thought imo.

> >. If you want to use nulls, well
> > mathematically your looking at a meta-language, and you simply can not
> > condense it all down into a single conceptual level (Or hofstadter
> > might point out that you have to pass it up to the next djinn!).
>
> What's that supposed to mean ?

That mathematical levels are being confused in this discussion (a very easy thing to do). Null talks about the underlying algebra - it is not part of it. As such it is the next mathematical layer, part of a meta language _about_ that level beneath it. You must not try to squash them in together. I recommend having a look at the development of set theory from naive to modern for a better description of this sort of thing than I can offer, or even better reading Hofstadters description of formal mathematical and logic systems in the early chapters of "Godel, Escher, Bach" - as opposed to a textbook, as hofstadter adds useful analogies which actually make the content readable :)

All best, Jim. Received on Fri Dec 09 2005 - 14:17:22 CET