Mysql查询Join两个表

Question

我正在编写一个简单的聊天服务。 我想选择两个人之间的消息线程，只有发件人和收件人使用他们的用户名或用户 ID。 我在下面有我的查询。 它有一个小问题，正在返回一条消息两次。 我怎样才能解决这个问题？

例如，如果我的发件人有 uid '101' 而我的收件人有用户名 'janed'，我的查询是

SELECT * FROM chat LEFT JOIN users ON sender=uid OR receiver=uid  WHERE username='janed' OR (sender='101' AND receiver='janed') OR (sender='janed' AND receiver='101') ORDER BY msg_id ASC

用户表

---------------------------------
uid |fname  |mname  |username   |
---------------------------------
101 |John   |Doe    |jdoe       |
---------------------------------
102 |Jane   |Doe    |janed      |
---------------------------------
103 |Ann    |Other  |aother     |
---------------------------------

聊天桌

---------------------------------------------
msg_id  |sender |receiver   |content        |
---------------------------------------------
1       |101    |102        | Hello Jane    |
---------------------------------------------
2       |102    |101        | Hello John    |
---------------------------------------------
3       |103    |101        | Hi John       |
---------------------------------------------

Answer 1

添加GROUP BY子句

SELECT 
    *
FROM
    chat
        LEFT JOIN
    users ON sender = uid OR receiver = uid
WHERE
    username = 'janed'
        OR (sender = '101' AND receiver = 'janed')
        OR (sender = 'janed' AND receiver = '101')
GROUP BY msg_id
ORDER BY msg_id ASC

Answer 2

SELECT
    u.*, c.*
  FROM Users u
  LEFT JOIN Chat c ON (
    c.sender = u.uid OR c.receiver = u.uid
  )
  WHERE u.uid = 101 OR u.uid = 102 /* and/or other conditions */
  GROUP BY c.msg_id
  ORDER BY c.msg_id ASC