Php 通过UNION group按给定字段连接2个表，并仅获取组中最新的表

我正在一个小仪表板中构建一个电子邮件收件箱。是使用其他API的自定义电子邮件客户端。我准备DB结构的方式如下：

输入的

表

╔════╦════════════════════╦═══════════════════╦═════════════════════════════════════════╦══════════════════════╗
║ id ║     email_from     ║     email_to      ║               email_body                ║         date         ║
╠════╬════════════════════╬═══════════════════╬═════════════════════════════════════════╬══════════════════════╣
║  1 ║ stack@overflow.com ║ mitch@example.com ║ Your question has a downvote for newbie ║ 2017-12-28T00:00:00Z ║
║  2 ║ stack@exchange.com ║ mitch@example.com ║ You gotta learn SQL, buddy :/           ║ 2017-12-28T00:01:43Z ║
║  3 ║ stack@overflow.com ║ mitch@example.com ║ Guess what? 42                          ║ 2017-12-28T00:05:00Z ║
║  3 ║ stack@overflow.com ║ mitch@example.com ║ This is a bot, stop responding          ║ 2017-12-28T00:10:00Z ║
╚════╩════════════════════╩═══════════════════╩═════════════════════════════════════════╩══════════════════════╝

╔════╦═══════════════════╦════════════════════╦════════════════════════════════════╦══════════════════════╗
║ id ║    email_from     ║      email_to      ║             email_body             ║         date         ║
╠════╬═══════════════════╬════════════════════╬════════════════════════════════════╬══════════════════════╣
║  1 ║ mitch@example.com ║ stack@overflow.com ║ That is bad news                   ║ 2017-12-28T00:00:50Z ║
║  2 ║ mitch@example.com ║ stack@exchange.com ║ I know :(                          ║ 2017-12-28T00:01:45Z ║
║  3 ║ mitch@example.com ║ stack@overflow.com ║ Answer to the Ultimate Question... ║ 2017-12-28T00:07:42Z ║
╚════╩═══════════════════╩════════════════════╩════════════════════════════════════╩══════════════════════╝

导出的
表

╔════╦════════════════════╦═══════════════════╦═════════════════════════════════════════╦══════════════════════╗
║ id ║     email_from     ║     email_to      ║               email_body                ║         date         ║
╠════╬════════════════════╬═══════════════════╬═════════════════════════════════════════╬══════════════════════╣
║  1 ║ stack@overflow.com ║ mitch@example.com ║ Your question has a downvote for newbie ║ 2017-12-28T00:00:00Z ║
║  2 ║ stack@exchange.com ║ mitch@example.com ║ You gotta learn SQL, buddy :/           ║ 2017-12-28T00:01:43Z ║
║  3 ║ stack@overflow.com ║ mitch@example.com ║ Guess what? 42                          ║ 2017-12-28T00:05:00Z ║
║  3 ║ stack@overflow.com ║ mitch@example.com ║ This is a bot, stop responding          ║ 2017-12-28T00:10:00Z ║
╚════╩════════════════════╩═══════════════════╩═════════════════════════════════════════╩══════════════════════╝

╔════╦═══════════════════╦════════════════════╦════════════════════════════════════╦══════════════════════╗
║ id ║    email_from     ║      email_to      ║             email_body             ║         date         ║
╠════╬═══════════════════╬════════════════════╬════════════════════════════════════╬══════════════════════╣
║  1 ║ mitch@example.com ║ stack@overflow.com ║ That is bad news                   ║ 2017-12-28T00:00:50Z ║
║  2 ║ mitch@example.com ║ stack@exchange.com ║ I know :(                          ║ 2017-12-28T00:01:45Z ║
║  3 ║ mitch@example.com ║ stack@overflow.com ║ Answer to the Ultimate Question... ║ 2017-12-28T00:07:42Z ║
╚════╩═══════════════════╩════════════════════╩════════════════════════════════════╩══════════════════════╝

所以，我想做一个类似Gmail的小收件箱，在这里你可以看到所有电子邮件的列表，按电子邮件地址（或对话）分组，按日期排序，在顶部显示最新消息，在每个对话中，只显示该对话的最后一条消息


我的查询尝试

SELECT `email_from` as `thread_email`, `email_body`, `date`
FROM (
SELECT `email_from`, `email_body`, `date` FROM `incoming`
UNION
SELECT `email_to` as `email_from`, `email_body`, `date` FROM `outgoing`
) AS t_union

GROUP BY `email_from`
ORDER BY `date` DESC

结果：

╔════════════════════╦═════════════════════════════════════════╦══════════════════════╗
║    thread_email    ║               email_body                ║         date         ║
╠════════════════════╬═════════════════════════════════════════╬══════════════════════╣
║ stack@exchange.com ║ You gotta learn SQL, buddy :/           ║ 2017-12-28T00:01:43Z ║
║ stack@overflow.com ║ Your question has a downvote for newbie ║ 2017-12-28T00:00:00Z ║
╚════════════════════╩═════════════════════════════════════════╩══════════════════════╝

它仅显示来自
传入
表的结果


我的预期结果

SELECT `email_from` as `thread_email`, `email_body`, `date`
FROM (
SELECT `email_from`, `email_body`, `date` FROM `incoming`
UNION
SELECT `email_to` as `email_from`, `email_body`, `date` FROM `outgoing`
) AS t_union

GROUP BY `email_from`
ORDER BY `date` DESC


按电子邮件地址分组，这表示对话（称为
thread\u email
，基本上每个
thread\u email
一行）
每行（或对话，或
thread\u email
）仅显示该
thread\u email
的最新消息（最后一条消息可能是传入的或传出的）
按日期排序（最近的第一个）

SQL Fiddle:

这是实现这一目标的新手方式吗？对另一种数据库结构有什么建议，这种结构更简单，但也更干净和有序吗？
正如Gordon所说，如果
group by
您的数据减少到一行，但如果您只使用
order by
子句，您仍然会得到重复的数据，它将从union表中获取所有数据

groupby
子句也有一个
having
子句，但它似乎只适用于数字，比如
count（*）>2
和其他内容

在中，我们有一个叫做
数组\u agg
的东西，它将一些数据转换成数组，然后我们可以作为数组访问它

在MySQL中，我发现的唯一相似之处是，使用与“，”连接的字符串中相同的grouped by语句转换所有数据，因此，idk如果这可以解决您的问题，但您可以尝试在应用程序端处理结果，而不是在SQL端处理

您可以这样使用：

select email_from, group_concat(email_body), date from (
  select email_from, email_body, date from incoming union all
  select email_to as email_from, email_body, date from outgoing
) as t_union group by email_from order by date desc;

或者，在最后一种情况下，以非优化的方式：

select email_from, email_body, date from (
  select email_from, email_body, date from incoming union all
  select email_to as email_from, email_body, date from outgoing
) as t_union_1 where date in (
  select max(date) from (
    select email_from, email_body, date from incoming union all
    select email_to as email_from, email_body, date from outgoing
  ) as t_union group by email_from order by date desc
);

为什么要用两个表而不是一个表呢？不完全是，我正在尽可能有条理地构建这个web应用程序。此外，我发现这种基于2表的结构更容易构建特定的对话页面（我的问题是构建对话的主索引）。你对改善我目前的结构有什么建议吗？非常感谢您的帮助：）最后一个案例似乎如预期的那样有效。我希望它能以10000张唱片表现出色。谢谢你，马克西米利安！