สอบถามการ sum ข้อมูลใน SQL มาแสดงครับ รบกวนหน่อยนะครับ

<?
session_start();
	if (session_is_registered("valid_uname") && session_is_registered("valid_pwd")) {
?>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<style type="text/css">
<!--
.style1 {
	font-family: "TH SarabunPSK";
	font-size: 18pt;
	font-weight: bold;
}
.style2 {
	font-family: "TH SarabunPSK";
	font-size: 16pt;
	font-weight: bold;
}
.style5 {cursor: hand; font-weight: normal; color: #000000;}
.style9 {font-family: Tahoma; font-size: 12px; }
.style11 {font-size: 12px}
.style13 {font-size: 9}
.style16 {font-size: 9; font-weight: bold; }
.style17 {font-size: 12px; font-weight: bold; }
-->
</style>
<?
require('connect.php');
?>
<form name="form1" action="<?php $_SERVER['PHP_SELF']?>">
<table align="center" width="200" border="0">
  <tr>
    <td><? require("head.php");?></td>
  </tr>
  <tr>
    <td bgcolor="#F8F8F8" align="center"><table  align="center"width="793" height="193" border="1" bgcolor="#F8F8F8">
      <tr>
        <td  class="style2"width="120" height="48">รายการปีงบประมาณ</td>
        <td  class="style2" colspan="5">ปีงบประมาณ
          <select name="datebg" onChange="this.form.submit()">
                    <option value="">เลือก</option>
      <?php
                    $sql = "SELECT * FROM tbbudgetyear";
                    $result = mysql_query($sql);
                    while($row = mysql_fetch_array($result)){
                    if($datebg == $row['BudgetyearID']){ $sel="selected"; }else{ $sel=""; }
                    ?>
                    <option value="<?=$row['BudgetyearID']?>" <?=$sel;?>><?=$row['Budgetyear']?></option>
                <?php } ?>
    </select>
         </td>
      </tr>
     
      <tr>
     	 <td height="28" align="center" class="style2">รหัส</td>
        <td height="28" colspan="2" align="center"  class="style2">หมวด</td>
        <td width="170" align="center" class="style2">จำนวนเงินที่ได้รับจัดสรร</td>
        <td width="213" align="center" class="style2">จำนวนเงินที่ใช้จริง</td>
        <td width="141" colspan="2" align="center" class="style2">จำนวนเงินคงเหลือ</td>
        </tr>
     <?php
            $sql_show  = "SELECT * FROM tbbudgetcategory";
           	$sql_show .= " LEFT JOIN tbbudgettype ON tbbudgetcategory.BudgettypeID = tbbudgettype.BudgettypeID";
			$sql_show .= " WHERE tbbudgetcategory.BudgetyearID = '".$datebg."'";
			$result_show = mysql_query($sql_show) or die(mysql_error());
			
            if(mysql_numrows($result_show) > 0){ // เช็คว่ามีข้อมูลหรือไม่
            while($row_show = mysql_fetch_array($result_show)) {
        ?>
      <tr class="style2">
      	<td height="24" align="center" class="style2"><?=$row_show['BudgetcategoryID']; ?></td>
        <td height="23" colspan="2" align="center"><?=$row_show['Budgettype']; ?></td>
        <td align="center"><?=number_format($row_show['Budgetcategoryamount'],2,'.',',');?></td>
        <td align="center">
		<?=number_format($row_show['Totalbill'],2,'.',',');?></td>
        <td colspan="2" align="center"><?=number_format($row_show['$Budgetcategoryamount=$Budgetcategoryamount-$Totalbill'],2,'.',',');?></td>
        </tr>
           <?php } } else { ?>
        <tr align="center">
            <td colspan="9"  class="style2">ไม่พบข้อมูลในปีงบประมาณนี้</td>
        </tr>
        <?php } ?>
      <tr>
        <td height="82" colspan="6" align="center"><img src="picture/back.gif" alt="" width="246" height="78" /></td>
        </tr>
    </table></td>
  </tr>
  <tr>
    <td><? require ('footer.php')?></td>
  </tr>
</table>
</form>
<?
	}
else{
 echo"<script language='JavaScript'>";
									echo"alert('Please Login');";
									echo"</script>";		
									echo "<meta http-equiv='refresh' content='0;url=index.php' />";
exit(); 
}
?>