在PostgreSQL中分组LIMIT:显示每个组的前N行? [英] Grouped LIMIT in PostgreSQL: show the first N rows for each group?

问题描述

我需要按照自定义列的顺序为每个组获取前N行.

I need to take the first N rows for each group, ordered by custom column.

给出下表:

db=# SELECT * FROM xxx;
 id | section_id | name
----+------------+------
  1 |          1 | A
  2 |          1 | B
  3 |          1 | C
  4 |          1 | D
  5 |          2 | E
  6 |          2 | F
  7 |          3 | G
  8 |          2 | H
(8 rows)

每个 section_id 我需要前两行(按 name 排序)，即结果类似于:

I need the first 2 rows (ordered by name) for each section_id, i.e. a result similar to:

 id | section_id | name
----+------------+------
  1 |          1 | A
  2 |          1 | B
  5 |          2 | E
  6 |          2 | F
  7 |          3 | G
(5 rows)

我正在使用PostgreSQL 8.3.5.

I am using PostgreSQL 8.3.5.

推荐答案

新解决方案(PostgreSQL 8.4)

New solution (PostgreSQL 8.4)

SELECT
  * 
FROM (
  SELECT
    ROW_NUMBER() OVER (PARTITION BY section_id ORDER BY name) AS r,
    t.*
  FROM
    xxx t) x
WHERE
  x.r <= 2;

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