SELECT *，LAST（date_time）

Question

我正在运行一个while循环来获取每个对话的最新消息，但是我收到错误

[18-Feb-2012 21:14:59] PHP Warning:  mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /path/messages.php on line 44

这是完整的循环。 它在我添加，LAST（date_time）和GROUP BY会话之前工作但是我只想在每个对话中显示最新消息，而不是每条消息。 我怎样才能做到这一点？

<?php 
  $current_user = user_info('id',1); //get logged in user id
  $query = mysql_query("SELECT *, LAST(date_time) 
                          FROM messages 
                         WHERE user_to='$current_user' 
                      ORDER BY date_time DESC 
                         LIMIT 0,5 
                      GROUP BY conversation");
  while($message = mysql_fetch_array($query)) { 
?>
                <li><strong><?php user_info('first_name',0,$message['user_from']); ?> <?php $sender_last_name = user_info('last_name',0,$message['user_from']); ?></strong><span class="message_time"><?php echo time2str($message['date_time']); ?></span><br />
                <span class="message_description"><?php echo substr($message['content'],0,50); ?></span></li>
                <?php } ?>

谢谢。

Answer 1

GROUP BY子句在WHERE之后，在ORDER BY子句之前：

  $query = mysql_query("SELECT *, LAST(date_time) 
                          FROM messages 
                         WHERE user_to='$current_user' 
                      GROUP BY conversation
                      ORDER BY date_time DESC 
                         LIMIT 0,5 ");

Answer 2

查看查询，您似乎在语句排序方面遇到了一些问题。

看看有关Select语句的MySQL手册 ：

SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [FROM table_references
    [WHERE where_condition]
    [GROUP BY {col_name | expr | position}
      [ASC | DESC], ... [WITH ROLLUP]]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position}
      [ASC | DESC], ...]
    [LIMIT {[offset,] row_count | row_count OFFSET offset}]
    [PROCEDURE procedure_name(argument_list)]
    [INTO OUTFILE 'file_name' export_options
      | INTO DUMPFILE 'file_name'
      | INTO var_name [, var_name]]
    [FOR UPDATE | LOCK IN SHARE MODE]]

GROUP BY语句应该在ORDER BY之前，然后是LIMIT 。 您在声明中的订购是错误的。

此外，就像Mike提到的那样，你应该使用mysql_error()来查看MySQL抛出的错误。