Re: Interpretation of Relations

From: Joe Thurbon <usenet_at_thurbon.com>
Date: Sat, 20 Jan 2007 11:21:54 GMT
Message-ID: <2007012021214043658-usenet@thurboncom>

On 2007-01-20 20:49:52 +1000, Bob Badour <bbadour_at_pei.sympatico.ca> said:

> 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 = <<>:{}>

Ah, cute.

So, just checking that I understand, DEE has all of its (one) possible tuples in its body, the tuple with no values.

Many thanks,
Joe Received on Sat Jan 20 2007 - 05:21:54 CST