Mysql查询Join两个表

Question

I am writing a simple chat service. I want to pick out a messages thread between two people only a sender and a recipient using either their usernames or user ids. I have my query below. It has a slight problem, is returning a message twice. How can I fix this?

For example if my sender has the uid '101' and my recipient has the username 'janed', my query is

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

Users table

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

Chat table

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

Answer 1

Add a GROUP BY clause

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