Re: Useful Oracle books - C.J. Date theory vs. practicality

Hmmm, I'm having trouble posting from the notebook...

From: "Lex de Haan" <lex.de.haan@xxxxxxxxxxxxxx>
> no problem. that's the only thing it's supposed to do ;-)

Yeah, that's why I tried to use IS NULL: to reduce what is at
the start a three-valued logic problem and bring it under
control of two-valued logic.

and I guess it is hard to make your workaround generic -- what if the
<condition> is constructed dynamically, at run time, and can e.g. be a
compound predicate with some ANDs and ORs?
then your "outer IF" becomes almost impossible to generate...


Sure.  But that was not the case of this workaround: it applies only
to PL/SQL "static" code.  Things get of course unstuck for dynamic
conditions.  Although ANDs and ORs I can't quite see as making it
necessarily more complex.  The problem IME is the three-valued
logic itself and how to handle it with two-value SQL logic.

The NVL and NVL2 functions in Oracle help across the board - in
fact the workaround I mentioned can be re-coded using it - but
it is still a kludge.  Essentially, one ends up associating
a particular outcome of three-valued logic with a specific value
in two-valued syntax.  I mean something like this:

(IF - or predicate) (nvl(<var>,'A') = 'something') (THEN yaddayadda)

Ie, replace the outer IF by a given *known* value check that
may (or may not) match a subsequent comparison.

Always very hard to handle three-valued logic without some formal
construct or syntax that can cover all options.  Which brings me
to my next question, if you'd be so kind:

Is there an agreed syntax (or language) anywhere that can hold
against three-valued logic?  As in: commercially available?
Os is three-valued logic condemned to forever be the
fly in the ointment (so to speak) of SQL?

--
Cheers
Nuno Souto
in sunny Sydney, Australia
dbvision@xxxxxxxxxxxxxxx
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