php - MYSQL 连接检索结果

Question

我会尽力解释我的问题。我正在使用 MYSQL/PHP 从两个表中获取数据，其结构如下所示:

     table A 
+---------+------------+
| userid  | username   |
+---------+------------+
|    1    |     john   |
|    2    |     doe    |
|    3    |     lewis  |
+---------+------------+

     table B
+---------+------------+-----------+
| id      |from_userid | to_userid |
+---------+------------+-----------+
|    1    |     1      |     3     |
|    2    |     3      |     2     |
|    3    |     1      |     2     |
|    4    |     2      |     1     |
+---------+------------+-----------+

Am trying to achieve the following:

+---------+------------+----------------------+
| id      |sender username| receiver username |
+---------+------------+----------------------+
|    1    |  john         |     lewis         |
|    2    |  lewis        |     doe           |
|    3    |  john         |     doe           |
|    4    |  doe          |     john          |
+---------+------------+----------------------+

如您所见，我没有返回发送者或接收者的用户 ID，而是根据表 A 返回他们的用户名。

在这种情况下我可以使用左连接还是右连接吗？提前致谢

Answer 1

试试这个

SELECT b.id, sender.username AS sender_username, receiver.username AS receiver_username
FROM tableB AS b
  JOIN tableA AS sender ON b.from_userid = sender.userid
  JOIN tableA AS receiver ON b.to_userid = receiver.userid