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Home -> Community -> Mailing Lists -> Oracle-L -> Re: Useful Oracle books - C.J. Date theory vs. practicality
> 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_at_optusnet.com.au
-- Archives are at http://www.freelists.org/archives/oracle-l/ FAQ is at http://www.freelists.org/help/fom-serve/cache/1.html -----------------------------------------------------------------Received on Sat May 29 2004 - 07:52:21 CDT
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