Re: Interpretation of Relations
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sat, 20 Jan 2007 10:49:52 GMT
Message-ID: <ktmsh.3018$1x.52150_at_ursa-nb00s0.nbnet.nb.ca>
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> Sorry, you've lost me there.
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> Can you do this? I thought that 'attributes' were effectively names of
> domains, so if there are no attributes, there are no domains from which
> you can "assign". What would it look like?
Date: Sat, 20 Jan 2007 10:49:52 GMT
Message-ID: <ktmsh.3018$1x.52150_at_ursa-nb00s0.nbnet.nb.ca>
Joe Thurbon wrote:
> On 2007-01-19 21:22:02 +1000, paul c <toledobythesea_at_oohay.ac> said:
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>> Joe Thurbon wrote:
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>>>
>>> Relation R_People = <<D_People>: {{Joe}}>
>>> Relation R_Hair Colour = <<D_People X D_Hair>: {{Joe, Blond}}>
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>>> (The bit in the <> is the relation header, the subsequent sets are
>>> the relation body).
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>>> R_Hair Colour: <<D_People X D_Hair>: {}>
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>>> indicates that I don't know the colour of Joe's hair. It really means
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>>> NOT Joes hair is Red
>>> NOT Joes hair is Blond
>>>
>>> Is this right? ...
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>> >> I think this is something that often trips me up - we can obtain >> tuples that have no attributes only by projecting away other >> attributes (which I believe is the purpose of TABLE_DEE and DUM)
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> Sorry, you've lost me there.
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>> or by declaring a relation that has no attributes and then "assigning" >> (basically, this means memorizing) a value that stands for true or >> false to that relation.
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> Can you do this? I thought that 'attributes' were effectively names of
> domains, so if there are no attributes, there are no domains from which
> you can "assign". What would it look like?
Using your notation:
Relation DEE = <<>:{{}}>
Relation DUM = <<>:{}>
Received on Sat Jan 20 2007 - 11:49:52 CET