如何加入/联合来自同一张表的多个结果？

Question

我的 mysql 表看起来像：

user_messages

标识 |用户 ID |电话号码 |留言 |方向 | created_at

用户

标识 |名字

我想将 user_messages 'group by' 两次，然后 UNION 结果。我为什么要这样做？因为 user_id 有时有一个有效的用户 id（除了'-1'）然后我按它分组，如果它有 -1，然后按 phone_number 分组。

如果 user_id 设置为有效用户，我还想将结果与 users 表左连接以获取用户名

我几乎完成了查询，但问题是：
- 我希望结果具有来自 group by 的记录是最新的，这意味着最大的 created_at 值

select * from (
(
    select *, count(*) as `total` from
      (select `user_id`, `message`, `created_at`, `phone_number`,`direction` from `users_messages` where `user_id` != -1  order by `created_at` desc)
    as `t1` group by `user_id`
)
union
(
    select *, count(*) as `total` from
        (select `user_id`, `message`, `created_at`, `phone_number`,`direction`  from `users_messages` where `user_id` = -1 order by `created_at` desc)
    as `t2` group by `phone_number`
)
) as `t3`
left join (select `id`,`name` from `users`) as `t4` on `t3`.`user_id` = `t4`.`id` order by `created_at` desc

这让我得到的是未按 created_at DESC 排序的结果

更新： 该查询实际上在我的本地机器上有效，但在生产服务器上无效。在我的本地机器上我有 5.5.42 - Source distribution 和在服务器上 Ver 14.14 Distrib 5.7.17, for Linux (x86_64) using EditLine wrapper ... 有什么问题？

在本地机器上，它正确地返回了 max created_at，但在服务器上，它返回了为按记录分组而创建的 FIRST

Answer 1

这样的事情应该可以工作：

SELECT s.`user_id`, um.`phone_number`, s.msgCount
   , um.`message`, um.`created_at`, um.`direction`
   , u.`name` AS userName
FROM (
    SELECT `user_id`, IF(`user_id` = -1, `phone_number`, '') AS altID, MAX(`created_at`) AS lastCreatedAt, COUNT(*) AS msgCount
    FROM `users_messages` 
    GROUP BY user_id, altID
) AS s
INNER JOIN `users_messages` AS um 
    ON s.user_id = um.user_id 
    AND s.altID = IF(um.`user_id` = -1, um.`phone_number`, '')
    AND s.lastCreatedAt = um.created_at
LEFT JOIN `users` AS u
    ON s.user_id = u.user_id
ORDER BY um.created_at DESC
;

• s 子查询获取每个用户和无用户电话号码的摘要信息；计算的摘要信息包括最新的created_at 值，用于以下用途....
• 与um 的联接获取最后一条消息的行数据（通过在联接条件中包含来自s 的lastCreatedAt 值）
• 与users 的最终连接用于获取已知用户的user.name（并假设没有-1 用户，或者这样的用户将具有适当的'unknown' 名称.)

Answer 2

由于您是按 user_id 和 phone_number 分组的，因此您无法保留消息或方向。在每个子查询中为 created_at 添加一个 max 函数。我认为这会奏效。

select * from (
(
    select user_id
          ,'' as phone_number
          ,max('created_at') as 'created_at'
          ,count(*) as `total` from
      (select `user_id`
             ,`created_at`
       from `users_messages` 
       where `user_id` != -1)
    as `t1` group by `user_id`
)
union
(
    select '' as user_id
          ,phone_number
          ,max('created at') as 'created_at'
          ,count(*) as `total` from
        (select `created_at`
               ,`phone_number'
         from `users_messages` 
         where `user_id` = -1)
    as `t2` group by `phone_number`
)
) as `t3`
left join (select `id`,`name` from `users`) as `t4` 
on `t3`.`user_id` = `t4`.`id` 
order by `created_at` desc