Select random row for each group

select distinct on (id) id, attributefrom like_thisorder by id, random()

If you only need the attribute column:

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

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$ mysqlUSE 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 rtestSELECT 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;