Re: Interpretation of Relations

Joe Thurbon wrote:

> 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 = <<>:{}>

Exactly! DEE is the relational equivalent of 'True' and is the identity element for JOIN while DUM is the relational equivalent of 'False'.

> Many thanks,
> Joe

You are very welcome! Received on Sat Jan 20 2007 - 15:17:46 CET