sum sql php ไม่เป็นคือจะเอา ผลรวมของ 2 ช่อง มาลบกันอะครับทำไง ผมมีตัวอย่าง

  <?
	$objDB = mysql_select_db("$dbname");
	$strSQL = "SELECT * FROM $tbname1 ";
	$objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
	$Num_Rows = mysql_num_rows($objQuery);

	$Per_Page = 50;   // Per Page

	$Page = $_GET["Page"];
	if(!$_GET["Page"])
	{
		$Page=1;
	}

	$Prev_Page = $Page-1;
	$Next_Page = $Page+1;

	$Page_Start = (($Per_Page*$Page)-$Per_Page);
	if($Num_Rows<=$Per_Page)
	{
		$Num_Pages =1;
	}
	else if(($Num_Rows % $Per_Page)==0)
	{
		$Num_Pages =($Num_Rows/$Per_Page) ;
	}
	else
	{
		$Num_Pages =($Num_Rows/$Per_Page)+1;
		$Num_Pages = (int)$Num_Pages;
	}


	$strSQL .=" order  by $id DESC LIMIT $Page_Start , $Per_Page";
	$objQuery  = mysql_query($strSQL);

	?>
<table width="50%" border="1" cellpadding="1" cellspacing="1">
	  <tr>
    <th> <div align="center">&#3623;&#3633;&#3609;/&#3648;&#3604;&#3639;&#3629;&#3609;/&#3611;&#3637;</div></th>
    <th> <div align="center">&#3619;&#3633;&#3610;</div></th>
    <th> <div align="center">&#3592;&#3656;&#3634;&#3618;</div></th>
    <th> <div align="center">&#3619;&#3634;&#3618;&#3585;&#3634;&#3619;</div></th>
	</tr>
	<?
	while($objResult = mysql_fetch_array($objQuery))
	{ 
	$bgc = ($bgc==$rowColor1) ? $rowColor2 : $rowColor1; 	
   	?> 
    <tr>
    <td bgcolor=<? echo "$bgc"; ?>><div align="center">
      <?=$objResult["$jj1"].'/'.$objResult["$jj2"].'/'.$objResult["$jj3"];?>
    </div></td>
    <td bgcolor=<? echo "$bgc"; ?>><div align="right">
      <?=number_format($objResult["$jj4"]);?>
    </div></td>
    <td bgcolor=<? echo "$bgc"; ?>><div align="right">
      <?=number_format($objResult["$jj5"]);?>
    </div></td>
    <td bgcolor=<? echo "$bgc"; ?>><?=$objResult["$jj6"];?></td>
    </tr>
	<?
	}
	?>
  </table>
	<br>

  Total   <?= $Num_Rows;?>   Record :   <?=$Num_Pages;?>   Page :
  <?
	if($Prev_Page)
	{
		echo " <a href='$_SERVER[SCRIPT_NAME]?Page=$Prev_Page&txtKeyword=$_GET[txtKeyword]'><< Back</a> ";
	}

	for($i=1; $i<=$Num_Pages; $i++){
		if($i != $Page)
		{
			echo "[ <a href='$_SERVER[SCRIPT_NAME]?Page=$i&txtKeyword=$_GET[txtKeyword]'>$i</a> ]";
		}
		else
		{
			echo "<b> $i </b>";
		}
									}
	if($Page!=$Num_Pages)
	{
		echo " <a href ='$_SERVER[SCRIPT_NAME]?Page=$Next_Page&txtKeyword=$_GET[txtKeyword]'>Next>></a> ";
	}
	
	?>