Re: NULLs: theoretical problems?

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:

>> >> 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
> 
>