Question

    SELECT u.id,
            'Shift' AS which,
             se.created_at
    FROM users AS u
    JOIN schedule_elements AS se ON se.owner_id = u.id
    UNION ALL
            (SELECT u.id,
                         'Like' AS which,
                         ll.created_at
            FROM users AS u
            JOIN likes AS ll ON ll.owner_id = u.id
            UNION
            SELECT u.id,
                         'Comment' AS which,
                         cm.created_at
            FROM users AS u
            JOIN comments AS cm ON cm.owner_id = u.id)
    ORDER BY ID DESC , created_at DESC


The output looks like:
id, which, created_at
555, shift, <date>
555, shift, <date>
555, comment, <date>
555, shift, <date>
555, like, <date>
333, shift, <date>
333, shift, <date>
333, comment, <date>
333, shift, <date>
111, like, <date>
111, shift, <date>
111, shift, <date>

输出有5个用于id 555的条目，4个用于333.我想过滤这个查询，这样我只有555的前3个条目，333的前3个等等。

Answer 1

使用限制条款https://www.postgresql.org/docs/current/static/queries-limit.html

    (SELECT u.id,'Shift' AS which,se.created_at
    FROM users AS u
    JOIN schedule_elements AS se ON se.owner_id = u.id ORDER BY u.id DESC , u.created_at DESC LIMIT 3)
    UNION
    (SELECT u.id,'Like' AS which,ll.created_at
    FROM users AS u
    JOIN likes AS ll ON ll.owner_id = u.id ORDER BY u.id DESC , u.created_at DESC LIMIT 3)
    UNION
    (SELECT u.id,'Comment' AS which,cm.created_at
    FROM users AS u
    JOIN comments AS cm ON cm.owner_id = u.id ORDER BY u.id DESC , u.created_at DESC LIMIT 3)

Answer 2

可能你需要这样的东西。对于CTE中的p，我使用您的查询。在p2中，我通过窗口函数为所有行设置row_number，之后只能显示每个用户的第一个/最后一个。

 WITH p AS (
    SELECT u.id,
            'Shift' AS which,
             se.created_at
    FROM users AS u
    JOIN schedule_elements AS se ON se.owner_id = u.id
    UNION ALL
    SELECT u.id,
           'Like' AS which,
             ll.created_at
    FROM users AS u
    JOIN likes AS ll ON ll.owner_id = u.id
    UNION ALL
    SELECT u.id,
         'Comment' AS which,
         cm.created_at
    FROM users AS u
    JOIN comments AS cm ON cm.owner_id = u.id
), p2 AS (
   SELECT *,
        row_number() OVER (PARTITION BY id ORDER BY created_at DESC) AS rank
   FROM p
   WHERE which IS NOT NULL
)
SELECT id, which, created_at --, rank
FROM p2
WHERE rank <= 3
ORDER BY ID DESC , created_at DESC;

Postgresql限制列为每个值3

2 个答案: