Random via Subselect or Temporary Table

[Thomas T. Thai]

I would like to select a random record from a group of records so I'd end
up with one random record per group:

CREATE TABLE randtest (
catnum int,
title varchar(32)
);

INSERT INTO randtest VALUES (1, 'one.one');
INSERT INTO randtest VALUES (1, 'one.two');
INSERT INTO randtest VALUES (1, 'one.three');
INSERT INTO randtest VALUES (1, 'one.four');
INSERT INTO randtest VALUES (1, 'one.five');
INSERT INTO randtest VALUES (2, 'two.one');
INSERT INTO randtest VALUES (2, 'two.two');
INSERT INTO randtest VALUES (2, 'two.three');
INSERT INTO randtest VALUES (2, 'two.four');
INSERT INTO randtest VALUES (2, 'two.five');
INSERT INTO randtest VALUES (3, 'three.one');
INSERT INTO randtest VALUES (3, 'three.two');
INSERT INTO randtest VALUES (3, 'three.three');
INSERT INTO randtest VALUES (3, 'three.four');
INSERT INTO randtest VALUES (3, 'three.five');

I've got it working with using temporary tables:

DROP table t1; DROP table t2;

CREATE TEMPORARY TABLE t1 AS SELECT catnum, title, random() AS r FROM
randtest;
CREATE TEMPORARY TABLE t2 AS SELECT catnum, max(r) AS mr FROM t1 GROUP BY
catnum;
SELECT t1.catnum, t1.title, t1.r FROM t1, t2 WHERE t1.catnum = t2.catnum
AND t1.r = t2.mr;
catnum | title | r
--------+-----------+-------------------
2 | two.one | 0.576068660046937
3 | three.one | 0.695552298191726
1 | one.one | 0.988770103076831

catnum | title | r
--------+-------------+-------------------
1 | one.one | 0.602969813907039
3 | three.three | 0.851651187451394
2 | two.two | 0.904609308067993

catnum | title | r
--------+-------------+-------------------
2 | two.three | 0.659522682269813
3 | three.three | 0.697027135964961
1 | one.two | 0.895630815949119

The results seem pretty random per group. Is there a better way of
accomplishing this task without using temporary tables? Maybe with
subselects?

Thanks,
Thomas



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[Tom Lane]

"Thomas T. Thai" <to*@minnesota.com> writes:

CREATE TEMPORARY TABLE t1 AS SELECT catnum, title, random() AS r FROM
randtest;
CREATE TEMPORARY TABLE t2 AS SELECT catnum, max(r) AS mr FROM t1 GROUP BY
catnum;
SELECT t1.catnum, t1.title, t1.r FROM t1, t2 WHERE t1.catnum = t2.catnum
AND t1.r = t2.mr; The results seem pretty random per group. Is there a better way of
accomplishing this task without using temporary tables? Maybe with
subselects?

I'd suggest using DISTINCT ON (catnum) with ORDER BY catnum, random().
See the SELECT reference page's "weather reports" example for
background.

regards, tom lane

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[Thomas T. Thai]

<quote who="Tom Lane">

I'd suggest using DISTINCT ON (catnum) with ORDER BY catnum, random().
See the SELECT reference page's "weather reports" example for
background.

Good suggestion. Thank you Tom.

Thomas

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