+----------------------+
| 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
最佳答案
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”。
关于Mysql左连接合并列,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14182221/