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Home -> Community -> Mailing Lists -> Oracle-L -> RE: SQL question
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.
-----Original Message-----
From: Henry Poras [mailto:henry_at_itasoftware.com]
Sent: Friday, December 03, 2004 21:04
To: lex.de.haan_at_naturaljoin.nl; jhn.aida_at_cbs.dk; geraldine_2_at_comcast.net
Cc: oracle-l_at_freelists.org
Subject: RE: SQL question
How about a line (pipe, HV, ...) with endpoints a and b?
Henry
-----Original Message-----
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]
On Behalf Of Lex de Haan
Sent: Friday, December 03, 2004 12:58 PM
To: jhn.aida_at_cbs.dk; geraldine_2_at_comcast.net
Cc: oracle-l_at_freelists.org
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.
-----Original Message-----
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]
On Behalf Of Jesper Haure Norrevang
Sent: Friday, December 03, 2004 18:07
To: geraldine_2_at_comcast.net
Cc: oracle-l_at_freelists.org
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
> 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
> --
> http://www.freelists.org/webpage/oracle-l
>
-- http://www.freelists.org/webpage/oracle-l -- http://www.freelists.org/webpage/oracle-l -- http://www.freelists.org/webpage/oracle-lReceived on Fri Dec 03 2004 - 14:27:29 CST
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