From: PFC <lists@peufeu.com>
To: Fay Du <fay.du@versaterm.com>
To: pgsql-sql@postgresql.org
Subject: Re: How can I selet rows which have 2 columns values cross equal?
Date: Fri, 10 Mar 2006 15:32:29 +0100
Message-ID: <op.s57b0fvbcigqcu@apollo13> (raw)
In-Reply-To: <001d01c64445$fec23350$b8d849c6@versaterm.com>
References: <001d01c64445$fec23350$b8d849c6@versaterm.com>
What are your conditions on a and b ? Can a be equal to b on a row ? If
so, do you want this row ?
If you want to avoid duplicates, I suggest first removing them, then
adding a constraint CHECK( a<b ) for instance. Then, from you r
application (or in an ON INSERT trigger), swap a and b if a>b.
I added some values to your table for completeness :
SELECT * FROM test;
a | b | id
-----+-----+----
100 | 101 | 1
101 | 100 | 2
100 | 3 | 3
20 | 30 | 4
11 | 13 | 5
3 | 33 | 6
30 | 20 | 7
666 | 666 | 8
666 | 666 | 9
500 | 666 | 10
666 | 500 | 11
123 | 123 | 12
456 | 789 | 13
456 | 789 | 14
Try :
SELECT x.id AS xid, y.id AS yid, x.a AS xa, x.b AS xb, y.a AS ya, y.b AS
yb FROM test x, test y WHERE x.a=y.b AND x.b=y.a;
xid | yid | xa | xb | ya | yb
-----+-----+-----+-----+-----+-----
1 | 2 | 100 | 101 | 101 | 100
2 | 1 | 101 | 100 | 100 | 101
4 | 7 | 20 | 30 | 30 | 20
7 | 4 | 30 | 20 | 20 | 30
8 | 9 | 666 | 666 | 666 | 666
8 | 8 | 666 | 666 | 666 | 666
9 | 9 | 666 | 666 | 666 | 666
9 | 8 | 666 | 666 | 666 | 666
10 | 11 | 500 | 666 | 666 | 500
11 | 10 | 666 | 500 | 500 | 666
12 | 12 | 123 | 123 | 123 | 123
You'll get 2 rows for each match. You can add a condition to remove the
dupes :
SELECT x.id AS xid, y.id AS yid, x.a AS xa, x.b AS xb, y.a AS ya, y.b AS
yb FROM test x, test y WHERE x.a=y.b AND x.b=y.a AND x.id<=y.id;
xid | yid | xa | xb | ya | yb
-----+-----+-----+-----+-----+-----
1 | 2 | 100 | 101 | 101 | 100
4 | 7 | 20 | 30 | 30 | 20
8 | 9 | 666 | 666 | 666 | 666
8 | 8 | 666 | 666 | 666 | 666
9 | 9 | 666 | 666 | 666 | 666
10 | 11 | 500 | 666 | 666 | 500
12 | 12 | 123 | 123 | 123 | 123
If you don't want the rows with a=b, replace x.id<=y.id with x.id<y.id
SELECT x.id AS xid, y.id AS yid, x.a AS xa, x.b AS xb, y.a AS ya, y.b AS
yb FROM test x, test y WHERE x.a=y.b AND x.b=y.a AND x.id<y.id;
xid | yid | xa | xb | ya | yb
-----+-----+-----+-----+-----+-----
1 | 2 | 100 | 101 | 101 | 100
4 | 7 | 20 | 30 | 30 | 20
8 | 9 | 666 | 666 | 666 | 666
10 | 11 | 500 | 666 | 666 | 500
It is going to be slow, though. Basically a full self join. Let's hack
this :
CREATE AGGREGATE array_accum (
sfunc = array_append,
basetype = anyelement,
stype = anyarray,
initcond = '{}'
);
I'll leave this one as an exercice to the reader ;)))
SELECT array_accum(id), CASE WHEN a<b THEN a ELSE b END AS low, CASE WHEN
a<b THEN b ELSE a END AS high FROM test GROUP BY low, high HAVING
sum(sign(a-b)) = 0 AND count(*)>1;
array_accum | low | high
-------------+-----+------
{10,11} | 500 | 666
{4,7} | 20 | 30
{1,2} | 100 | 101
{8,9} | 666 | 666
On Fri, 10 Mar 2006 14:24:44 +0100, Fay Du <fay.du@versaterm.com> wrote:
> Hi All:
> I have a table like this:
>
> Table test
> Id | a | b
> -----+----+---
> 1 | 100| 101
> 2 | 101| 100
> 3 | 100| 3
> 4 | 20 | 30
> 5 | 11 | 13
> 6 | 3 | 33
> 7 | 30 | 20
>
> I want to get row 1, 2,4 and 7 selected. Because their values of a and b
> are cross equal. i.e., for each pair of rows,
> a.Row1 = b.Row2 and b.Ro1 = a.Row2
> a.Row4 = b.Row7 and b.Ro4 = a.Row7
>
> How can I construct a sql statement to select them?
> Thanks in advance.
> Fay
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster
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To: pgsql-sql@postgresql.org
Cc: lists@peufeu.com, fay.du@versaterm.com
Subject: Re: How can I selet rows which have 2 columns values cross equal?
In-Reply-To: <op.s57b0fvbcigqcu@apollo13>
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