# Re: NULLs: theoretical problems?

Date: Thu, 23 Aug 2007 21:00:11 -0000

Message-ID: <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.

- Jan Hidders