Re: boolean datatype ... wtf?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 29 Sep 2010 12:45:40 -0300
Message-ID: <4ca35f20$0$14812$9a566e8b_at_news.aliant.net>
>>> value, not a relation."
>>
>>
>> An algebra is a set of values and a set of operations closed on that set
>> of values.
>>
>> Date is saying that comparisons are not part of the relational algebra
>> in the same way that division is not part of the integer algebra because
>> dividing any two arbitrary integers is not necessarily closed on
>> integers.
>>
>> Consider an expression of the form:
>>
>> R = f(A,B)
>>
>> where both A and B are relations and f is some binary operation. If f is
>> natural join, then R is a relation. If f is a comparison, then R is not
>> a relation; it is a boolean.
>> ...
Date: Wed, 29 Sep 2010 12:45:40 -0300
Message-ID: <4ca35f20$0$14812$9a566e8b_at_news.aliant.net>
paul c wrote:
> On 29/09/2010 7:45 AM, Bob Badour wrote:
>
>> Paul Mansour wrote:
>>
>>> On Sep 29, 9:46 am, paul c <toledobythe..._at_oohay.ac> wrote:
>>>
> ... >>>> part of the relational algebra-- because their result is a truth
>>>> Aren't A=B and A>B relations?
>>>
>>>
>>> I was under the impression that they are not, even if A and B are
>>> relations. But I may have misintepreted C.J. Date on this.
>>>
>>> In "Database in Depth" he writes:
>>>
>>> "In Chapter 2, I mentioned the fact that the equality comparison
>>> operator "=" applies to every type. In partitular, therefore it
>>> applies to relation types.... Now I must immediatly explain that these
>>> opeartors are not relational operators as such -- that is they are not
>>> value, not a relation."
>>
>>
>> An algebra is a set of values and a set of operations closed on that set
>> of values.
>>
>> Date is saying that comparisons are not part of the relational algebra
>> in the same way that division is not part of the integer algebra because
>> dividing any two arbitrary integers is not necessarily closed on
>> integers.
>>
>> Consider an expression of the form:
>>
>> R = f(A,B)
>>
>> where both A and B are relations and f is some binary operation. If f is
>> natural join, then R is a relation. If f is a comparison, then R is not
>> a relation; it is a boolean.
>> ...
> > Thanks for the precise example. It seems to me that Date means your > last sentence as his starting point. Nothing 'wrong' with that, but my > question might be 'why does that have to be so'? Isn't it just as valid > to say that R can be one of DEE or DUM for certain f?
Sure, but that's a different f. The first f is part of a closed boolean algebra, and the second f is not. Received on Wed Sep 29 2010 - 17:45:40 CEST