使用UNION时，ORDER BY和LIMIT不起作用

Question

这是我的疑问：

mysql_query("SELECT userid FROM messages WHERE userid<>'$myid' AND friendid='$myid' AND status=1 UNION SELECT friendid FROM messages WHERE userid='$myid' AND friendid<>'$myid' AND status=1");

我想添加分页，所以我需要限制我发送消息的朋友数量。但每当我添加LIMIT时，它都显示错误。另外，我想首先显示最新消息ORDER BY time DESC，但这也不起作用。帮忙？

Answer 1

将该查询包含在子查询中，将ORDER BY ... LIMIT...包含在外部查询中：

SELECT *
FROM
(
    SELECT userid 
    FROM messages 
    WHERE userid   <> '$myid' 
      AND friendid = '$myid' 
      AND status = 1 
    UNION 
    SELECT friendid 
    FROM messages
    WHERE userid = '$myid' 
      AND friendid <> '$myid' 
      AND status = 1
) AS sub
ORDER BY ...
LIMIT ...;

Answer 2

尝试一下

SELECT DISTINCT userid FROM...