mysql - 如何获取我在消息系统中与之交互的用户列表

Question

我正在尝试获取与我在消息系统中交互过的用户列表:

表:

messages: id, sender_id, recipient_id, message, date

SQL:

SELECT * FROM `messages` 
WHERE 
(`sender_id` = :user_id OR `recipient_id` = :user_id)
GROUP BY `sender_id`, `recipient_id`
ORDER by `date` DESC

这给了我多行发件人/收件人 ID 不匹配的

我认为是 GROUP BY 导致的，不确定如何修复它

编辑:每次交互我只需要 1 行(因此 GROUP BY )

Answer 1

GROUP BY 将合并具有相同 sender_id 和 recipient_id 的所有行。使用 UNION 将两个组合并为一组结果:

SELECT correspondent, MAX(date) AS last_date
FROM (
    SELECT sender_id as correspondent, date
    FROM messages
    WHERE recipient_id = :user_id

    UNION ALL

    SELECT recipient_id as correspondent, date
    FROM messages
    WHERE send_id = :user_id) x
GROUP BY correspondent
ORDER BY last_date DESC