Re: NULLs: theoretical problems?

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