Re: NULLs: theoretical problems?

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 fact, I have no idea what you mean with that phrase. I can think of a few meanings but none of those is what SQL does.

• Jan Hidders