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   |
+----------+---------+-------------------------+

我有两个表名post和post_replies 表post_id中的post_replies在表post中为Fk到Pk 我想要做的是找到帖子消息，它只回复一列

如果我这样运行：

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

它将返回类似这样的内容

+-----------+-------------------------+
| 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   |
+-----------+-------------------------+

有没有办法可以像这样获得结果：

+-------------------------+
|   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

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

无论如何都可能吗？

任何帮助非常感谢

Answer 1

UNION您提供的两个查询并介绍正确的顺序：

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

如果您不想要重复，请将“UNION ALL”更改为“UNION”。

Answer 2

我知道这不是你想要的，但只是发帖如果能解决你的问题

$查询

SELECT 
    group_concat(pr.message SEPARATOR '|'), p.message,  
FROM 
    `posts` AS p 
LEFT JOIN 
    post_replies AS pr ON p.post_id = pr.post_id 
GROUP BY
    pr.post_id


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

Mysql左连接和合并列

2 个答案: