Select random row for each group

Question

I have a table like this

ID    ATTRIBUTE
 1    A
 1    A
 1    B
 1    C
 2    B
 2    C
 2    C
 3    A
 3    B
 3    C

I\'d like to selec

Answer 1

Put a big random number in front of each record (id) and choose within each group the record with the lowest random number.

$ cat test.txt
\N  1   a
\N  2   b
\N  2   c
\N  2   d
\N  3   e
\N  4   f


$ mysql

USE test;
DROP TABLE test;
CREATE TABLE test (id0 INT NOT NULL AUTO_INCREMENT, id VARCHAR(1),  attribute VARCHAR(1), PRIMARY KEY (id0));
LOAD DATA LOCAL INFILE '~/mysql/test.txt' INTO TABLE test FIELDS TERMINATED BY '\t';

DROP TABLE rtest;
CREATE TABLE rtest (random INT(8), id0 VARCHAR(1), id VARCHAR(1),  attribute VARCHAR(1),  PRIMARY KEY (id, random));

INSERT INTO rtest
SELECT CAST(1000000. * rand() AS INT) AS random, test.* FROM test;

SELECT rtest.* FROM rtest,
(SELECT id, min(random) AS random FROM rtest GROUP BY id) AS sample WHERE rtest.random=sample.random AND rtest.id=sample.id;

Answer 2

select distinct on (id) id, attribute
from like_this
order by id, random()

If you only need the attribute column:

select distinct on (id) attribute
from like_this
order by id, random()

Notice that you still need to order by id first as it is a column of the distinct on.

If you only want the distinct attributes:

select distinct attribute
from (
    select distinct on (id) attribute
    from like_this
    order by id, random()
) s