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From: aaron@grad2.cis.upenn.edu (Aaron Watters)
Newsgroups: comp.databases
Subject: Re: Outerjoin implementation? (4-valued question)
Message-ID: <30712@netnews.upenn.edu>
Date: 7 Oct 90 20:23:37 GMT
References: <5803@plains.NoDak.edu> <PCG.90Sep18192004@odin.cs.aber.ac.uk> <CIMSHOP!DAVIDM.90Sep19100915@uunet.UU.NET> <PCG.90Sep24152920@odin.cs.aber.ac.uk> <30203@netnews.upenn.edu> <CIMSHOP!DAVIDM.90Sep30013925@uunet.UU.NET> <152008@felix.UUCP>
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Reply-To: aaron@grad2.cis.upenn.edu.UUCP (Aaron Watters)
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One variety of 4 valued logic is unknown/true/false/error

eg.

`bush is president' is true.
`dukakis is president' is false.
`all space aliens are green' is unknown.
`the king of North America is bald' is error.

unknown means `this may be true or false or error, but we don't
  know which.'

error means `this can't have any truth value' -- in the example
  above because any truth value requires the existence of someone
  who (last I heard) doesn't exist.		-aaron