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@ursa-nb00s0.nbnet.nb.ca>

Joe Thurbon wrote:
> On 2007-01-19 21:22:02 +1000, paul c <toledobythesea_at_oohay.ac> said:
>

>> Joe Thurbon wrote:

>
>
> ...
>

>>>
>>> Relation R_People      = <<D_People>: {{Joe}}>
>>> Relation R_Hair Colour = <<D_People X D_Hair>: {{Joe, Blond}}>
>>>
>>> (The bit in the <> is the relation header, the subsequent sets are 
>>> the relation body).

>
>
> ...
>

>>>
>>>
>>> R_Hair Colour: <<D_People X D_Hair>: {}>
>>>
>>> indicates that I don't know the colour of Joe's hair. It really means
>>>
>>> NOT Joes hair is Red
>>> NOT Joes hair is Blond
>>>
>>> Is this right? ...

>
>
> ...
>

>>
>> 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)

>
>
> Sorry, you've lost me there.
>

>> 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.

>
>
> 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 - 04:49:52 CST