Re: Two-valued logic
Date: Tue, 30 Dec 2003 07:25:51 GMT
Message-ID: <3M9Ib.11648$lo3.1065_at_newsread2.news.pas.earthlink.net>
Dawn M. Wolthuis wrote:
> I work with a model that uses a two-valued logic. A NULL value under this
> scenario can be handled logically as a null set value. With this model, a
> NULL then = a NULL because a null set equals a null set..
Is your use of the indefinite article ('a two-valued logic') significant - that is, does the model you work with use one of a number of possible two-valued logics, but by implication not the familiar Boolean (Aristotelean) logic? (I'm just nit-picking, unless the answer is that 'a' is significant - but then I'm very puzzled!)
> My impression from reading Date and others is that the three-valued logic of
> SQL that is proliferated in RDBMS's does not have a lot of fans. However, I
> don't know if that is really the case or if I just happen to be reading the
> pro-two-valued logic folks.
Date would argue that SQL DBMS are not RDBMS (or, at any rate, not
TRDBMS, where the T stands for True). However, that's nit-picking, again.
I don't like 3-valued logic; it is harder to do right than 2-valued
logic. There are undeniably occasions when the value represented in
SQL as a NULL would be better represented by an empty set.
If you want to find a proponent of 3-valued logic, look at some of Don
Chamberlain's books on DB2/UDB -- at least one of them contains a
defense of 3-valued logic as found in SQL, and it is moderately
clearly a riposte to some of Date's arguments.
> Who is out there that will still defend three-valued logic within databases
> and suggest that it is a better strategy than using a two-valued approach?
Not me.
> Would it be accurate to state that most database theorists agree that a
The proponents of 3-valued logic argue that the performance benefits
claimed for 2-valued logic over 3-valued logic are over-stated. I've
not done the experimentation to justify that. The clarity of 2-valued
logic is better than 3-valued logic. The rules (and things like
tautologies) are more familiar, so people would probably make fewer
mistakes using a 2-valued logic. And when an empty set is appropriate
as the 'unknown' answer, using an empty set instead of a third logic
> two-valued logic provides significant benefits?
-- Jonathan Leffler #include <disclaimer.h> Email: jleffler_at_earthlink.net, jleffler_at_us.ibm.com Guardian of DBD::Informix v2003.04 -- http://dbi.perl.org/Received on Tue Dec 30 2003 - 08:25:51 CET