Php 在mysql中处理日期？

我有一个很长的mysql查询，我试图找到比给定日期更早的行

这是我使用的代码：

$result="SELECT
               u.username, u.picture,m.id, m.user_note, m.reply_id, m.reply_name, m.recycle_id, m.recycle_name, m.dt
              FROM
                relationships r,
                notes m,
                user u
              WHERE
                m.user_id = r.leader
              AND
                r.leader = u.user_id
              AND
                r.listener = ".$_SESSION['user_id']."
             UNION
               select username, picture,id, user_note, reply_id, reply_name, recycle_id, recycle_name, dt
               from user u, notes b
               where u.user_id = b.user_id
               AND
               b.user_id =".$_SESSION['user_id']."
               AND
        WHERE dt < '$lastmsg'
               ORDER BY dt DESC LIMIT 10";

mysql_query($result) or die(mysql_error().$result);

和
其中dt<“$lastmsg”

---

这是无效的语法。删除“WHERE”。

如果

$lastmasg

=

2010-09-20 12:53:43'

多余的引号将导致语法错误。mysql报告的错误是什么？我刚刚编辑了我的问题，错误更大！

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'where dt <'2010-09-20 12:53:43' ORDER BY dt DESC LIMIT 10' at line 20SELECT u.username, u.picture,m.id, m.user_note, m.reply_id, m.reply_name, m.recycle_id, m.recycle_name, m.dt FROM relationships r, notes m, user u WHERE m.user_id = r.leader AND r.leader = u.user_id AND r.listener = 2 UNION select username, picture,id, user_note, reply_id, reply_name, recycle_id, recycle_name, dt from user u, notes b where u.user_id = b.user_id and b.user_id =2 and where dt <'2010-09-20 12:53:43' ORDER BY dt DESC LIMIT 10

           AND
    WHERE dt < '$lastmsg'