Re: NULLs: theoretical problems?

From: V.J. Kumar <vjkmail_at_gmail.com>
Date: Thu, 23 Aug 2007 23:27:31 +0200 (CEST)
Message-ID: <Xns9995B19A4E450vdghher@194.177.96.26>

Jan Hidders <hidders_at_gmail.com> wrote in news:1187902811.569206.68690_at_r23g2000prd.googlegroups.com:

> On 23 aug, 00:13, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

>> Jan Hidders <hidd..._at_gmail.com> wrote
>> innews:1187811230.504947.11400_at_x40g2000prg.googlegroups.com: 
>>
>> > On 22 aug, 17:37, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:
>> >> 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'.
>>
>> I am not sure I understand what you mean by "That's not exactly the
>> same...", but I'll take a stab at it.

>
> I mean that if you take a select-from-where query with a formula f in
> in the where clause containing the DEF construct then replacing f with
> f' where all the DEF constructs are removed (i.e. we replace "DEF x :
> g" with "g" until there are no more such expressions) sometimes
> changes the meaning of the query.

Let's take a look at it. Could you give a query example and you interpretration of the DEF construct therein ?

>
> -- Jan Hidders
>
>
Received on Thu Aug 23 2007 - 16:27:31 CDT