Re: Useful Oracle books - C.J. Date theory vs. practicality
- From: "Nuno Pinto do Souto" <dbvision@xxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Sat, 29 May 2004 22:54:16 +1000
----- Original Message -----
From: "Lex de Haan" <lex.de.haan@xxxxxxxxxxxxxx>
> no problem. that's the only thing it's supposed to do ;-) and it is purely
> two-valued.
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 the 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 is purely
reserved for PL/SQL "static" code. Things get of course unstuck
for dynamic conditions. Although ANDS and ORs I can't see as making
it necessarily more complex. The problem IMHO is the three-valued logic
itself and how to handle it with two-valued 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.
I mean something like this:
(IF - or predicate) (nvl(var,'A') = 'something') then etcetc
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?
Or is three-valued logic condemned to forever be the fly in the
ointment (so to speak) of SQL?
Cheers
Nuno Pinto do Souto
dbvision@xxxxxxxxxxxxxxx
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