Mysql左连接合并列

Question

+----------------------+
| post_id | message    |
+----------------------+
| 1       | message 1  |
+----------------------+
| 2       | message 2  |
+----------------------+

+----------+---------+-------------------------+
| reply_id | post_id |message                  |
+----------+---------+-------------------------+
|    1     |    1    |  reply 1 to message 1   |
+----------+---------+-------------------------+
|    2     |    1    |  reply 2 to message 1   |
+----------+---------+-------------------------+
|    3     |    1    |  reply 3 to message 1   |
+----------+---------+-------------------------+
|    4     |    2    |  reply 1 to message 2   |
+----------+---------+-------------------------+

我得到了这两个表名 post 和 post_replies表 post_replies 中的 post_id 是表 post 中的 Fk 到 Pk我想做的是找到帖子消息，它只回复一栏

如果我运行这样的程序:

SELECT p.message, pr.message FROM `posts` AS p LEFT JOIN post_replies AS pr ON p.post_id = pr.post_id

它会返回这样的东西

+-----------+-------------------------+
| message   |   message               |
+-----------+-------------------------+
| message 1 |  reply 1 to message 1   |
+-----------+-------------------------+
| message 1 |  reply 2 to message 1   |
+-----------+-------------------------+
| message 1 |  reply 3 to message 1   |
+-----------+-------------------------+
| message 2 |  reply 1 to message 2   |
+-----------+-------------------------+

有没有办法得到这样的结果:

+-------------------------+
|   message               |
+-------------------------+
|   message 1             |
+-------------------------+
|  reply 1 to message 1   |
+-------------------------+
|  reply 2 to message 1   |
+-------------------------+
|  reply 3 to message 1   |
+-------------------------+
|   message 2             |
+-------------------------+
|  reply 1 to message 2   |
+-------------------------+

如果我跑

SELECT p.message FROM `posts` AS p LEFT JOIN post_replies AS pr ON p.id = pr.post_id

这将从post 表返回消息

或者

SELECT pr.message FROM `posts` AS p LEFT JOIN post_replies AS pr ON p.id = pr.post_id

这将仅从表 post_replies 返回消息

有可能吗？

非常感谢任何帮助

Answer 1

联合您提供的两个查询并引入正确的顺序:

SELECT * FROM (
  SELECT p.id, 0 x, p.message
  FROM posts AS p
  LEFT JOIN post_replies AS pr ON p.id = pr.post_id
  UNION ALL
  SELECT p.id, pr.id, pr.message
  FROM posts AS p
  LEFT JOIN post_replies AS pr ON p.id = pr.post_id
) x
ORDER BY 1, 2

如果您不想重复，请将“UNION ALL”更改为“UNION”。