php - 通过给定字段通过 UNION 组加入 2 个表，并仅获取该组的最新信息

Question

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:

传入表

╔════╦════════════════════╦═══════════════════╦═════════════════════════════════════════╦══════════════════════╗
║ 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 ║
╚════╩════════════════════╩═══════════════════╩═════════════════════════════════════════╩══════════════════════╝

传出表

╔════╦═══════════════════╦════════════════════╦════════════════════════════════════╦══════════════════════╗
║ 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，我想做一个像 Gmail 这样的小收件箱，您可以在其中看到所有电子邮件的列表，按电子邮件地址(或对话)分组并按日期排序，在顶部显示最新的，在每个对话中，显示仅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 incoming 表的结果。

---

我的预期结果

• 按电子邮件地址分组，这代表一个对话(称为 thread_email，基本上每个 thread_email 一行)
• 每一行(或 session ，或 thread_email)仅显示该 thread_email 的最新消息(最后一条消息可能是传入或传出)
• 按日期排序(最近的在前)

╔════════════════════╦════════════════════════════════╦══════════════════════╗
║    thread_email    ║           email_body           ║         date         ║
╠════════════════════╬════════════════════════════════╬══════════════════════╣
║ stack@overflow.com ║ This is a bot, stop responding ║ 2017-12-28T00:10:00Z ║
║ stack@exchange.com ║ I know :(                      ║ 2017-12-28T00:01:45Z ║
╚════════════════════╩════════════════════════════════╩══════════════════════╝

SQL fiddle :http://sqlfiddle.com/#!9/ed9642/4

这是实现这一目标的新手方法吗？对另一种数据库结构有什么建议，这种结构更简单但也更清晰有序？

Answer 1

正如 Gordon 所说，如果 group by 你的数据，将数据减少到一行，但如果你只使用 order by 子句，你仍然会得到重复的数据, 它将从联合表中获取所有数据。

group by 子句也有一个 having 子句，但它似乎只适用于数字，比如 count(*) > 2 和其他东西。

在PostgreSQL我们有一个叫做 array_agg 的东西，它将一些数据转换为数组，然后我们可以将其作为数组访问。

在 MySQL 中，我发现的唯一类似的是 group_concat将所有数据与相同的分组语句转换为字符串中用'，'连接的语句，所以，idk如果这解决了你的问题，但你可以尝试这个并在应用程序端处理结果，而不是在SQL端处理。

你可以这样使用:

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，以非优化方式执行:

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
);