Re: The Relational Model & Queries That Naturally Return Duplicate Rows

On Oct 9, 11:58 pm, "Joe Thurbon" <use..._at_thurbon.com> wrote:
> On Sun, 10 Oct 2010 13:23:50 +1000, Brian <br..._at_selzer-software.com>  
> wrote:
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> > On Oct 9, 10:56 pm, "Joe Thurbon" <use..._at_thurbon.com> wrote:
> >> On Sun, 10 Oct 2010 08:36:38 +1000, Erwin <e.sm..._at_myonline.be> wrote:
> >> > On 10 okt, 00:14, Cimode <cim..._at_hotmail.com> wrote:
>
> >> >> Getting an unintuitive point across to somebody who
> >> >> probably never read any significant RM theory material
>
> >> > Hmmmmmmmm. Now I'm just being curious.
>
> >> > How can you match this claim of yours with the fact that the OP
> >> > explicitly states in his question that
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> >> > "Since this is not a relation - because it contains duplicate rows -
> >> > ".
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> >> > Do you think there exists any source _BUT_ "significant RM theory
> >> > material" where he could have acquired the knowledge to state that ?
>
> >> > I'm also curious as to why he's asking to be pointed out what I think
> >> > should be obvious, but then again recent discussions on other subjects
> >> > have revealed to me that there is no predicting when people will fail
> >> > to see the obvious, merely because of certain kinds of mental block
> >> > they happen to be suffering from.  I'm granting the OP the benefit of
> >> > the doubt that he might be suffering from a mental block of the kind
> >> > "surely if SQL is so widespread, then everything it does must be
> >> > right".
>
> >> In the context of recent discussion on the ttm mailing list regarding  
> >> ORDER BY, it's not clear to me that correct answer is indeed obvious.  
> >> (In fact, given my track record on cdt, I'm not sure that I am thinking  
> >> of the correct answer).
>
> >> With that caveat in mind, given that, with the OP's database, the query
>
> >> SELECT FIRST_NAME from USERS order by FIRST_NAME
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> >> presents the user with something that is not a relation, it seems  
> >> strange to require that
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> >> SELECT LAST_NAME from USERS
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> >> should. (Apart from that projection is a relational operator, and order  
> >> by isn't, but that seems to me at least to be slightly subtle).
>
> >> Cheers,
> >> Joe- Hide quoted text -
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> >> - Show quoted text -
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> > SELECT is not the same thing as projection, even if the columns in the
> > select list are a proper subset of the columns in the table.  It
> > follows that SELECT does not necessarily return a relation, even
> > without an ORDER BY.  SELECT DISTINCT is analogous to taking a
> > projection over the select list.
>
> Indeed. Thanks.
>
> Please revise my parenthetical to read:
>
> (Apart from that SELECT has, in projection, a relational operator that is  
> a close analogue, and ORDER BY doesn't, but ...)
>
> Of course, my above post presupposes that Erwin's and Cimode's 'obvious  
> answer' to the OP's question is that SELECT should behave in the same way  
> as SELECT DISTINCT. (Is that what you'd choose, too?)
>
> Cheers,
> Joe- Hide quoted text -
>
> - Show quoted text -

No. That's not what I'd choose. I would choose instead to keep both. Information is lost when transforming SELECT to SELECT DISTINCT. Received on Sun Oct 10 2010 - 08:25:15 CEST