Re: 4 the FAQ: Are Commercial DBMS Truly Relational?
Date: Sun, 10 Oct 2004 10:35:38 -0400
Message-ID: <pbhbkc.5d7.ln_at_mercury.downsfam.net>
Marshall Spight wrote:
> "Christopher Browne" <cbbrowne_at_acm.org> wrote in message
> news:2srmk1F1njk29U1_at_uni-berlin.de...
>>
>> If you cut columns off of the result set, it is possible for the
>> result set to, in fact, not be a "set", but rather a non-unique "bag"
>> of tuples.
>> [...]
>> That is NOT going to be a "set" or a "relation" if some customer made
>> multiple purchases between those dates.
>>
>> The problem that this expresses is that the relational algebra does
>> not satisfy the property of closure.
>
> That's an odd viewpoint.
>
> The math books I've read have made the point that when you
> are talking about sets, then {2, 2} is the same set as {2}.
> So if you have duplicates after a project, you just throw
> them away; they "don't count" so to speak.
>
Therefore, wouldn't the implementation of SQL have some sort of implied DISTINCT on every SELECT statement?
-- Kenneth Downs Use first initial plus last name at last name plus literal "fam.net" to email meReceived on Sun Oct 10 2004 - 16:35:38 CEST