Path: utzoo!attcan!uunet!vtserf!creatures!csgrad!holliday
From: holliday@csgrad.cs.vt.edu (Glenn Holliday)
Newsgroups: comp.databases
Subject: Re: errors and 4 valued logic
Summary: logic vs practical dbs
Keywords: logic, truth values
Message-ID: <694@creatures.cs.vt.edu>
Date: 6 Nov 90 01:17:30 GMT
Sender: usenet@creatures.cs.vt.edu
Reply-To: holliday@csgrad.cs.vt.edu (Glenn Holliday)
Distribution: na
Organization: Virginia Tech Computer Science, Blacksburg, VA
Lines: 40

Chuck Phillips  writes:
>Clairification: A statement requiring the existance of the non-existant to
>be true, is simply false, not an error.  However, a domain violation _is_
>an error.  (e.g. "I am green years old.")

while aaron@grad2.cis.upenn.edu (Aaron Watters) argues

>`the king of North America is bald' is error.
>if ``The king of North America is bald'' is false then we infer
>that ``The king of North America has hair'' must be true, right?
>... My favorite
>version of 4-valued logic resolves this difficulty by calling
>both statements `overdefined' or `erroneous.'

>PS: I also claim that `I am green years old' can reasonably be
>treated as false.

	I think this is another example of Neat vs Scruffy, otherwise known
as "What does logic _mean_" vs "How does the database work?".  The examples
can be logically represented to model any domain you like.  Whether you
want them to be false or errors depends completely on what domain you
define.  If any object can be in the domain, you can infer true statements
or fail to infer them, which makes them false (_if_ you define your logical
system to give that meaning to false!).
	On the other hand, practical database systems want to work with
simple domains and decide efficiently whether to allow a statement or
query.  Calling either example an error is legitimate as long as your
database system behaves consistently and tells the user why it rejected the
construct.
	To call both of these false, your database wants rules that say
1. The domain for every object/entity must be completely defined.
2. Any purported value which does not fall within the appropriate domain is
   reported as an error.
3. Any reference to a value which does not exist (but would fall within an
   appropriate domain if it did exist) is reported as an error.

	Of course, much of the hue and cry over null vs empty, etc is about
the best way to implement these rules.

Glenn Holliday  holliday@csgrad.cs.vt.edu