สอบถามปัญหา Search แบบ Dropdown List Error ถ้าใส่ ORDER BY DESC ครับ

<form name="frmSearch" method="GET" action="<?=$_SERVER['SCRIPT_NAME'];?>">
<?php
$strSQL = " SELECT * FROM tb_gadget,tb_department,tb_priority,tb_issue,tb_status
where
tb_gadget.Department_user = tb_department.Department_id
AND
tb_gadget.priority_tb = tb_priority.priority_id
AND 
tb_gadget.issue_tb = tb_issue.issue_id
AND 
tb_gadget.status_tb = tb_status.status_id ORDER BY Gadget_id DESC";
if (isset($_GET['ddlSelect'])!= "" and  $_GET["txtKeyword"]  != '')
	{
	  $strSQL .= " AND (".$_GET["ddlSelect"]." LIKE '%".$_GET["txtKeyword"]."%' ) ";
	}	
	$objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
	?>
 <table width="Auto" border="1">
   <tr>
     <th height="50"> <span class="col-sm-0"> </span>
       <span class="col-sm-3">
         <select class="form-control" name="ddlSelect"  placeholder="moของผู้ใช้งานอุปกรณ์" required id="ddlSelect">
           	   <option value="Department" <?if($_GET["ddlSelect"]=="Department"){echo"selected";}?>>Department</option>
               <option value="priority_type" <?if($_GET["ddlSelect"]=="priority_type"){echo"selected";}?>>PRIORITY</option>
               <option value="issue_type" <?if($_GET["ddlSelect"]=="issue_type"){echo"selected";}?>>ISSUE</option>
               <option value="name_user" <?if($_GET["ddlSelect"]=="name_user"){echo"selected";}?>>Name</option>
               <option value="Gadget_id" <?if($_GET["ddlSelect"]=="Gadget_id"){echo"selected";}?>>TICKET_ID</option>
         </select>
         </span><span class="col-sm-3">
         <input name="txtKeyword" type="text" placeholder="Search" id="txtKeyword" value="<?=$_GET["txtKeyword"];?>"onFocus="this.value = '';">
         </span><span class="col-sm-1">
         <button type="submit" class="btn btn-primary"> <span class="glyphicon glyphicon-search">.Search</span> </button>
         </span></th>
   </tr>
 </table>