[英]Mysql query to Join two tables
我正在編寫一個簡單的聊天服務。 我想選擇兩個人之間的消息線程,只有發件人和收件人使用他們的用戶名或用戶 ID。 我在下面有我的查詢。 它有一個小問題,正在返回一條消息兩次。 我怎樣才能解決這個問題?
例如,如果我的發件人有 uid '101' 而我的收件人有用戶名 'janed',我的查詢是
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
用戶表
---------------------------------
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 |
---------------------------------------------
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
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
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