获得不同用户的邮件数据库表

Question

对于messages表结构如下

id, sender_id, receiver_id, text 

我想建立不同的用户列表（由信息ID订购）谁要么发出了一个信息或接收一个来自当前会议的持有者。该列表只能包含一次用户。

的查询如：

"SELECT * FROM messages WHERE sender_id = '$sess_id' OR receiver_id = '$sess_id'"; 

产生重复即，与同一用户的多个行。是否必须在SQL查询之后进行筛选，或者是否存在会产生期望的唯一列表的SQL查询？

谢谢！

Answer 1

你可以这样做：

SELECT sender_id 
FROM messages 
WHERE receiver_id = '$sess_id' 
UNION 
SELECT receiver_id 
FROM messages 
WHERE sender_id = '$sess_id' 

Answer 2

您也可以使用不同的关键字：

SELECT distinct sender_id 
FROM messages 
WHERE receiver_id = '$sess_id' 
UNION 
SELECT distinct receiver_id 
FROM messages 
WHERE sender_id = '$sess_id' 

更多信息可以在这里找到：

http://dev.mysql.com/doc/refman/5.0/en/distinct-optimization.html

Answer 3

SELECT 
    CASE receiver_id WHEN '$sess_id' THEN sender_id ELSE receiver_id END AS user_id 
FROM messages 
WHERE receiver_id = '$sess_id' 
    OR sender_id = '$sess_id' 
GROUP BY CASE receiver_id WHEN '$sess_id' THEN sender_id ELSE receiver_id END 
ORDER BY MAX(message_id) 

MySQL允许您引用列别名在GROUP BY子句中，所以你也可以简单地这样做：

… 
GROUP BY user_id 
…