mysql - 私有(private)消息系统。列出每个对话的最后一条消息

Question

假设这是数据库结构:

SELECT * FROM `pms` where id_to = 1 or id_from = 1

这将返回他收到或发送的所有消息，

那么我如何从用户 1 可能拥有的每个对话中检索最后一条消息？

PD:当两个用户之间有一条或多条消息时，我称之为对话

-编辑-

因此给定此数据库内容:

我们想得到id 4和6

Answer 1

假设 id 是一个自增列:

SELECT MAX(id) AS id
FROM pms
WHERE id_to = 1 OR id_from = 1
GROUP BY (IF(id_to = 1, id_from, id_to))

假设您有 id_from 和 id_to 索引，这种变体很可能会执行得更好，因为 MySQL 不知道如何处理 OR:

SELECT MAX(id) AS id FROM
(SELECT id, id_from AS id_with
FROM pms
WHERE id_to = 1
UNION ALL
SELECT id, id_to AS id_with
FROM pms
WHERE id_from = 1) t
GROUP BY id_with

以下是获取这些 ID 的消息的方法:

SELECT * FROM pms WHERE id IN
    (SELECT MAX(id) AS id FROM
    (SELECT id, id_from AS id_with
    FROM pms
    WHERE id_to = 1
    UNION ALL
    SELECT id, id_to AS id_with
    FROM pms
    WHERE id_from = 1) t
    GROUP BY id_with)