php mysql 连接发送和接收的消息

Question

私人消息系统显示串联的用户消息。

• 用户发送和接收的消息按 time_sent 排序
• 每个消息选项卡将显示发送者/接收者的姓名、消息和发送时间
• 有关如何提高查询速度（如索引等）的任何提示？

MESSAGES TABLE 
--------------
id sender receiver time_sent message opened recipientDelete senderDelete

Users Table
-----------
id first_name last_name

我的尝试：

$userid = logged in user
$query="SELECT MAX(id), sender, receiver, MIN(time_sent), message, opened 
        FROM messages 
        WHERE (receiver='$userid' AND recipientDelete='0') 
             OR (sender='$userid' AND senderDelete='0') 
        GROUP BY receiver,sender 
        ORDER BY time_sent DESC LIMIT 8";

结果/问题：
如果登录用户 (id=3) 收到来自两个用户 (id=1,and id=2) 的消息，则此查询将返回来自用户 1 和 2 的消息

第二次尝试：

 $query="SELECT MAX(id), sender, receiver, MIN(time_sent), message, opened      
     FROM ( SELECT 
              CASE WHEN sender = '$userid' THEN receiver ELSE sender END
              MIN(time_sent)
            FROM messages 
            WHERE sender = '$userid' OR receiver = '$userid' 
            GROUP BY CASE WHEN sender = '$userid' THEN receiver ELSE sender END)
         ORDER BY time_sent DESC 
         LIMIT 8";

返回错误。

Answer 1

您正在从派生表中进行选择。在 MySQL 中，该子查询必须是别名：

SELECT ...
FROM ( SELECT ...) AS subqueryalias
                   ^^^^^^^^^^^^^^^^
WHERE ...