如何 select 一列中每个不同的元素只有两行 (PostgreSQL 9.3.5)

Question

现在我有一个看起来像这样的查询。 %s 用于 psycopg2:

SELECT DISTINCT ON (p1.creating_user_id) p1.post_id, p1.message
    FROM posts p1
    LEFT OUTER JOIN post_relations pr1 ON pr1.post_id=p1.post_id AND pr1.receiving_user_id=%s
    WHERE p1.creating_user_id IN (SELECT ur2.user_b_id 
        FROM user_relations AS ur2 
        WHERE ur2.user_a_id=%s 
        AND ur2.friend=true)
    ORDER BY p1.creating_user_id, p1.created_utc DESC

我如何将此查询更改为 return 两行以达到 creating_user_id 而不是只有一行？有没有办法在仍然使用 SELECT DISTINCT 的同时做到这一点，还是我必须做某种子查询？

Answer 1

一个window function可以做到

select post_id, message
from (
    select
        p1.post_id, p1.message,
        row_number() over(
            partition by p1.creating_user_id
            order by p1.created_utc desc
        ) as rn
    from
        posts p1
        left outer join
        post_relations pr1 on pr1.post_id = p1.post_id and pr1.receiving_user_id = %s
    where p1.creating_user_id in (
        select user_b_id 
        from user_relations
        where user_a_id = %s and friend = true
    )
) s
where rn <= 2

如何 select 一列中每个不同的元素只有两行 (PostgreSQL 9.3.5)

How to select only two rows for each distinct element in a colum (PostgreSQL 9.3.5)

postgresql