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Re: Two-valued logic

From: Jonathan Leffler <jleffler_at_earthlink.net>
Date: Tue, 30 Dec 2003 07:25:51 GMT
Message-ID: <3M9Ib.11648$lo3.1065@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!)

I don't dispute that null sets are equal. What does puzzle me is how you are storing a null set (as an alternative to some sort of NULL marker) in a single-valued attribute. Or are all attributes in relations actually set valued?

> 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
> two-valued logic provides significant benefits?

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 value would be a lot more sensible.

-- 
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 - 01:25:51 CST

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