Thread: 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.poluga@cores.co.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 |
7666 | 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
On Fri, Mar 10, 2006 at 08:24:44 -0500, 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. 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@lerctr.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 specifiedby LIMIT. This is because LIMIT is applied first. The command selects the specified number of rows, but mightthen block trying to obtain lock on one or more of them. Once the SELECT unblocks, the row might have been deletedor 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 | 100002 | 000003 | 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