Re: two nasty schemata, union types and surrogate keys
Date: 20 Nov 2009 22:52:52 GMT
>On Nov 8, 6:37 pm, r..._at_raampje.lan (Reinier Post) wrote:
>> Brian wrote:
>> >Under the closed world interpretation, there are no unknown truth
>> >values, but under the open world interpretation, only what has been
>> >explicitly asserted is known to be true, and it is known to be true
>> Yes ...
>> >even if the user that made the assertion is mistaken.
>> Huh?! So if I have a database relation 'X works at Y', with the open
>> world assumption, if someone updates the relation to say that Jane Doe
>> works at Acme Corp., then I must assume this to be true even if that
>> person is mistaken? How does that make any sense?
>Yes. If you don't also record who uttered the assertion, then the
>supposition implicit in its representation in the database under the
>closed world interpretation is that the assertion is true or under the
>open world interpretation is that whoever uttered the assertion knows
>that it is true. This makes sense because it is not known that the
>person is mistaken. If a bank teller counts his drawer at the end of
>the day and is neither over nor short, then the assumption must be
>that every customer received correct change, but it could be that one
>customer received a dollar more than he should have and another
>received a dollar less.
This is all very clear. What puzzles me is your use of the word 'known'. But I don't seem to make any progress trying to understand it so let's drop it.
-- ReinierReceived on Fri Nov 20 2009 - 23:52:52 CET