sql - 为每组选择随机行

Question

我有一张这样的桌子

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

我想为每个 ID选择一个随机属性。因此，结果可能如下所示（尽管这只是众多选项之一

ATTRIBUTE
B
C
C

这是我对这个问题的尝试

SELECT
  "ATTRIBUTE"
FROM
  (
  SELECT
    "ID",
    "ATTRIBUTE",
    row_number() OVER (PARTITION BY "ID" ORDER BY random()) rownum
  FROM
    table
  ) shuffled
WHERE
  rownum = 1

但是，我不知道这是否是一个好的解决方案，因为我需要引入行号，这有点麻烦。

你有更好的吗？

Answer 1

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

如果只需要属性列：

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

请注意，您仍然需要先排序，id因为它是distinct on.

如果您只想要不同的属性：

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

Answer 2

在每条记录（id）前面放一个大的随机数，并在每组中选择具有最低随机数的记录。

$ 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;