RE: SQL question
- From: "Lex de Haan" <lex.de.haan@xxxxxxxxxxxxxx>
- To: "'Henry Poras'" <henry@xxxxxxxxxxxxxxx>, <jhn.aida@xxxxxx>, <geraldine_2@xxxxxxxxxxx>
- Date: Fri, 3 Dec 2004 21:29:34 +0100
That's an interesting thought.
Now, the next question becomes: have these lines (pipes, ...) a direction?
A vector from point A to point B is not the same as a vector from point B to
point A...
But if (as apparently in this case) A and B can freely be swapped, because
there is no direction involved,
I guess it makes sense to define some ordering in the column values and
always put the smaller value in A and the larger one in B -- just like we do
in mathematics, and in time interval notations. We never talk about the
interval [42,13] but always about [13,42]. This translates into a relatively
easy row-level constraint: A less than or equal to B.
Obviously, for this to work you need "some ordering" in the column
population.
Cheers,
Lex.
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-----Original Message-----
From: Henry Poras [mailto:henry@xxxxxxxxxxxxxxx]
Sent: Friday, December 03, 2004 21:04
To: lex.de.haan@xxxxxxxxxxxxxx; jhn.aida@xxxxxx; geraldine_2@xxxxxxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Subject: RE: SQL question
How about a line (pipe, HV, ...) with endpoints a and b?
Henry
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Lex de Haan
Sent: Friday, December 03, 2004 12:58 PM
To: jhn.aida@xxxxxx; geraldine_2@xxxxxxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Subject: RE: SQL question
Hah -- comments ... Thanks for challenging me, Jesper :-) Well, my first
thought (when I saw this question) was: what are these columns A and B
supposed to mean in real world, to make this type of questions "reasonable"
in the first place? In other words, aren't we looking at a design mistake
here? How can you have two columns that apparently have precisely the same
meaning?
And another question: what is the primary key of this table?
Relational, relational, ... Isn't a table just like a spreadsheet, with rows
and columns? :-)
Undskyld, undskyld,
Lex.
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Tom Kyte Seminar: http://www.naturaljoin.nl/events/seminars.html
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-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Jesper Haure Norrevang
Sent: Friday, December 03, 2004 18:07
To: geraldine_2@xxxxxxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Subject: Re: SQL question
Geraldine,
select a, b
from mytable
where a <= b
union
select b, a
from mytable
where b = a
In Set Theory the set (1, 2) is equal to (2, 1).
Relational databases are (more and less) good implementations of Set Theory.
Basicly you are asking for a Set of Sets. Quite interesting. May be Lex has
some comments on this?
Regards
Jesper Haure
----- Original Message -----
From: geraldine_2@xxxxxxxxxxx
Date: Friday, December 3, 2004 5:37 pm
Subject: SQL question
> Hi,
> I have the following table below
>
> SQL> select * FRom mytable;
> A B
> ---------- ----------
> 1 2
> 3 4
> 2 1
> 5 6
> 4 3
>
> 5 rows selected.
>
> and I like to get the following output:
>
> A B
> ---------- ----------
> 1 2
> 3 4
> 5 6
>
>
> basically (1,2) is the same as (2,1) and I would just like to display
> any of those combination just once.
>
> Not sure how I can write a SQL to extract the data. Can someone help.
>
> TIA.
>
> Geraldine
> --
> //www.freelists.org/webpage/oracle-l
>
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