# Re: NULLs: theoretical problems?

Date: Wed, 22 Aug 2007 19:33:50 -0000

Message-ID: <1187811230.504947.11400_at_x40g2000prg.googlegroups.com>

On 22 aug, 17:37, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

*> Jan Hidders <hidd..._at_gmail.com> wrote innews:1187787815.215103.100820_at_k79g2000hse.googlegroups.com:
**>
**>
**>
*

> > On 22 aug, 13:23, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

*> >> Jan Hidders <hidd..._at_gmail.com> wrote
**> >> innews: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 ?
**>
**> > Yes, it is what line 3 shows, but I would not describe that as
**> > "substituting 'false' for unknown".
**>
**> In your language, the expression 'def y:x AND y' where 'y' is
**> 'undefined' evaluates to 'false'. In SQL, the expression 'x AND y' where
**> 'y'is 'unknown' evaluates to 'unknown'. The effect of having a predicate
**> that evaluates to 'unknown' is the same as having a predicate that
**> evaluates to 'false': no rows will be selected. That's what I meant by
**> "substituting 'false' for unknown".
*

That's not exactly the same because there are formulas f(x) that evaluate to 'true' if x is 'unknown'.

- Jan Hidders