Re: NULLs: theoretical problems?
Date: Thu, 23 Aug 2007 23:27:31 +0200 (CEST)
Jan Hidders <hidders_at_gmail.com> wrote in news:1187902811.569206.68690_at_r23g2000prd.googlegroups.com:
>> 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 - 23:27:31 CEST