如果列不存在，Mysql选择另一个表

我想从我的DB创建XML。数据保存在不同的表中。这就是为什么我想从不同的表列中选择，如果不存在于表中。

我如何写Select来检查表是否有我需要的列?

$sql = "SELECT products_quantity FROM ".$config['table_name'], $config['table_name2'].products_description;

这是完整的代码

<?php
//database configuration
$config['mysql_host'] = "localhost";
$config['mysql_user'] = "root";
$config['mysql_pass'] = "root";
$config['db_name']    = "sqltoxml";
$config['table_name'] = "products";
$config['table_name2'] = "products_description";

//connect to host
mysql_connect($config['mysql_host'],$config['mysql_user'],$config['mysql_pass']);
//select database
@mysql_select_db($config['db_name']) or die( "Unable to select database");
$xml          = "<?xml version='"1.0'" encoding='"UTF-8'"?>";
$root_element = $config['table_name']."s";
$xml         .= "<$root_element>";
//select all items in table
$sql = "SELECT products_id FROM ".$config['table_name'], products_description FROM ".$config['table_name2'];

$result = mysql_query($sql);
if (!$result) {
    die('Invalid query: ' . mysql_error());
}
if(mysql_num_rows($result)>0)
{
   while($result_array = mysql_fetch_assoc($result))
   {
      $xml .= "<".$config['table_name'].">";
      //loop through each key,value pair in row
      foreach($result_array as $key => $value)
      {
         //$key holds the table column name
         $xml .= "<$key>";
         //embed the SQL data in a CDATA element to avoid XML entity issues
         $xml .= "<![CDATA[$value]]>";
         //and close the element
         $xml .= "</$key>";
      }
      $xml.="</".$config['table_name'].">";
   }
}
    //close the root element
$xml .= "</$root_element>";
//send the xml header to the browser
header ("Content-Type:text/xml");
//output the XML data
echo $xml;
$fn=  "export.xml";
$fp = fopen($fn,"wb");
$write = fwrite($fp,$xml);
fclose($fp);
?>