# Re: NULLs: theoretical problems?

Date: Wed, 22 Aug 2007 13:23:52 +0200 (CEST)

Message-ID: <Xns99944B4879649vdghher_at_194.177.96.26>

Jan Hidders <hidders_at_gmail.com> wrote in news:1187766113.827952.167510_at_i38g2000prf.googlegroups.com:

> On 22 aug, 00:06, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

>> Jan Hidders <hidd..._at_gmail.com> wrote

>> innews:1187729150.610272.117790_at_r29g2000hsg.googlegroups.com:

>> >> I do not understand. You have:

*>>
**>> >> DEF y y DEF y:y
**>> >> 1 1 1 (1)
**>> >> 1 0 0 (2)
**>> >> 0 0 (3)
**>>
**>> >> So 'DEF y:y' will give the same result when y is either undefined
**>> >> or 'false', rows (2) and (3). How is it not substituting 'false'
**>> >> for undefined ?
**>>
**>> > In the way that if y is undefined then "DEF y : f" is not always
**>> > equivalent with "f[y/false]" i.e. "f" with all free occurrence of y
**>> > replaced with "false".
**>>
**>> I do not understand. Could you show what you mean with an example ?
*

>

> If y is undefined then "DEF y : NOT(y)" evaluates to "false".

Is it not what line(3) shows and what SQL queries do, namely substituting 'false' for unknown ? I say when y is undefined, the formula evaluates to 'false'; you say when y is undefined the formula evaluates to 'false'. I must be missing something !

> > -- Jan Hidders > >Received on Wed Aug 22 2007 - 13:23:52 CEST