Re: How can I selet rows which have 2 columns values cross equal?

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

On Friday 10 March 2006 14:24, Fay Du wrote:
>    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
>

select a.*
from test a, test b
where a.a = b.b and a.b = b.a
order by id

Regards, Milorad Poluga

--
---------------------------------------
Milorad Poluga
HK CORES Beograd, Makenzijeva 31
milorad(dot)poluga(at)cores(dot)co(dot)yu
---------------------------------------

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(dot)du(at)versaterm(dot)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

On Fri, Mar 10, 2006 at 08:24:44 -0500,
Fay Du <fay(dot)du(at)versaterm(dot)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.

Self join the table.
Something like (untested):

SELECT t1.id, t1.a, t1.b
FROM test t1, test t2
WHERE
t1.a = t2.b
AND
t1.b = t2.a
ORDER BY t1.id
;

Hi all,

I need to mark a row with a value in a column, but first i need to select
the first row without this mark. But in some concurrents cases i mark the
row twice. How can i lock the row to avoid others session get it?

TABLE TICKET
TICKET_NUMBER | MARK
00001 | 1
00002 | 0

I need to select the first row with 0 in MARK column and then mark it with
1.

regards,
Flavio Suguimoto

Flavio Suguimoto wrote:
> Hi all,
>
> I need to mark a row with a value in a column, but first i need to
> select the first row without this mark. But in some concurrents cases
> i mark the row twice. How can i lock the row to avoid others session
> get it?
>
> TABLE TICKET
> TICKET_NUMBER | MARK
> 00001 | 1
> 00002 | 0
>
> I need to select the first row with 0 in MARK column and then mark it
> with 1.

look at the FOR UPDATE in the select docs.

--
Larry Rosenman http://www.lerctr.org/~ler
Phone: +1 512-248-2683 E-Mail: ler(at)lerctr(dot)org
US Mail: 430 Valona Loop, Round Rock, TX 78681-3893

On Fri, Mar 10, 2006 at 02:57:51PM -0600, Larry Rosenman wrote:
> Flavio Suguimoto wrote:
> > I need to mark a row with a value in a column, but first i need to
> > select the first row without this mark. But in some concurrents cases
> > i mark the row twice. How can i lock the row to avoid others session
> > get it?
> >
> > TABLE TICKET
> > TICKET_NUMBER | MARK
> > 00001 | 1
> > 00002 | 0
> >
> > I need to select the first row with 0 in MARK column and then mark it
> > with 1.
>
> look at the FOR UPDATE in the select docs.

If you use FOR UPDATE with LIMIT then see the following in the
SELECT documentation:

It is possible for a SELECT command using both LIMIT and FOR
UPDATE/SHARE clauses to return fewer rows than specified by
LIMIT. This is because LIMIT is applied first. The command
selects the specified number of rows, but might then block
trying to obtain lock on one or more of them. Once the SELECT
unblocks, the row might have been deleted or updated so that
it does not meet the query WHERE condition anymore, in which
case it will not be returned.

For example, suppose you have the following data:

ticket_number | mark
---------------+------
00001 | 1
00002 | 0
00003 | 0
(3 rows)

Two concurrent transactions, T1 and T2, both run the following
query:

SELECT * FROM ticket WHERE mark = 0 LIMIT 1 FOR UPDATE;

One transaction, say T1, will get the following row:

ticket_number | mark
---------------+------
00002 | 0
(1 row)

T2 will block until T1 ends. If T1 rolls back or doesn't update
the row then T2 will get the above row. But if T1 updates the row
so that mark = 1 and then commits, then T2 will get an empty result
set instead of getting the next row with mark = 0. The queries
could use LIMIT 2 instead of LIMIT 1 and update only the first row
that came back, but then you'd have the same problem with a third
concurrent transaction (and with LIMIT 3 and a fourth transaction,
and so on).

--
Michael Fuhr

On Mar 10, 2006, at 22:24 , Fay Du wrote:

> 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

You need to use subqueries:

create table test
(
id integer primary key
, a integer not null
, b integer not null
);

copy test (id, a, b) from stdin;
1 100 101
2 101 100
3 100 3
4 20 30
5 11 13
6 3 33
7 30 20
\.

select t1.id as t1_id, t2.id as t2_id
from test t1
join test t2 on (t1.a = t2.b and t1.b = t2.a);
t1_id | t2_id
-------+-------
7 | 4
4 | 7
2 | 1
1 | 2
(4 rows)

And if you don't want to have each pair listed twice, just add WHERE
t1.a < t2.a, e.g.,

select t1.id as t1_id, t2.id as t2_id
from test t1
join test t2 on (t1.a = t2.b and t1.b = t2.a)
where t1.a < t2.a;
t1_id | t2_id
-------+-------
4 | 7
1 | 2
(2 rows)

Hope this helps!

Michael Glaesemann
grzm myrealbox com

On Mar 11, 2006, at 16:46 , Michael Glaesemann wrote:

> select t1.id as t1_id, t2.id as t2_id
> from test t1
> join test t2 on (t1.a = t2.b and t1.b = t2.a)
> where t1.a < t2.a;
> t1_id | t2_id
> -------+-------
> 4 | 7
> 1 | 2
> (2 rows)

Just a follow-up (mostly to myself): I've been toying with using
natural joins recently, and here's the same query rewritten to use a
natural join:

select id as t1_id, t2_id
from test t1
natural join (
select id as t2_id
, a as b
, b as a
from test
) t2
where id < t2_id;
t1_id | t2_id
-------+-------
4 | 7
1 | 2
(2 rows)

Michael Glaesemann
grzm myrealbox com