# Re: NULLs: theoretical problems?

Date: Sat, 25 Aug 2007 13:18:17 -0000

Message-ID: <1188047897.426229.297190_at_q4g2000prc.googlegroups.com>

On 25 aug, 01:53, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:

> Jan Hidders wrote:

*> > On 24 aug, 16:35, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:
**>
**> >>Jon Heggland <jon.heggl..._at_idi.ntnu.no> wrote innews:famjo6$i68$1_at_orkan.itea.ntnu.no:
**>
**> >>>In other words, the DEF query is equivalent to
**>
**> >>>SELECT * FROM t WHERE t.a IS NOT NULL AND (t.a = 5 OR TRUE)
**>
**> >>My understanding was that the DEF as written was a shorthand:
**>
**> >>e.g.
**> >>'def(x):((x or y) and x)' -> '((def(x) or y) and def(x))'
**>
**> >>You may be right, but then why the formula was not written with an
**> >>explicit 'and' ?
**>
**> > Because it does not satisify all the logical laws of an AND, so to
**> > avoid confusion another notation is used. Since it's related to an
**> > existential quantifier a similar notation was chosen (think of EXISTS
**> > x : f).
**>
**> > And, yes, as Jon correctly remarked it is in fact equivalent with
**> > something you can already write in SQL so you could achieve the same
**> > with some self-discipline.
**>
**> Let me suggest the use of 6th normal form and then EXISTS is all one
**> would need.
*

Why does everybody keep thinking that normal forms say something about null values!?(*) ;-) But I of course understand what you mean, and yes, that is all one would need, but that doesn't necessarily mean we cannot think about trying to go a little beyond the bare necessities.

(*) If they do then it is moste likely already 1NF that does so since it defines what a relation is and that definition would normally not include null values.

- Jan Hidders