include_once('conn.php');
$result = mysql_query("SELECT p_class FROM Product group by p_class limit 40");//第一查询，已经能成功获得数据
 mysql_data_seek($result, 0);
    while ($row=mysql_fetch_row($result))
    {
      for ($i=0; $i<mysql_num_fields($result); $i++ )
      {
       $tmp=$row[$i]; //把查询结果赋值为新变量：$tmp
       echo $tmp;//这里已经可以在web段显示正确的字段内容

       $result2 = mysql_query("SELECT * FROM Product where p_class=$tmp limit 0,4");//开始嵌套查询
       while ($row=mysql_fetch_row($result2))
    {    
            echo $row['pic'];     //这里查不到数据
            echo $row['p_class']; //同上
            echo $row['p_name'];  //同上
        }
      } 
    }   
mysql_free_result($result);
mysql_free_result($result2);
mysql_close($conn);