ขอความช่วยเหลือครับ จะเพิ่ม SUM(amount) as s_total ิอย่างไรจึงไม่ error ครับ

<?php
$sql_show = "select ots_table.*,SUM(amount) as s_Total,charge_by_table.percent,driver_table.Group_Fleet FROM ots_table,charge_by_table,driver_table";
	if($_POST['date_start']) {  
		$date_start = $_POST['date_start'];
		$date_stop = $_POST['date_stop']; 
		$sql_show .=" WHERE (pickup_start_date between '".$_POST['date_start']."' AND '".$_POST['date_stop']."') 
		 and ots_table.driverid like driver_table.driverid 
	     and ots_table.chargedby like charge_by_table.charge_by
		 ORDER BY pickup_start_date, Group_Fleet ASC";
    }
	$result_show = mysql_query($sql_show) or die(mysql_error());
	$chk_date = false;  
	$chk_sum = 0;
	$total = 0;
	while($rs = mysql_fetch_array($result_show)) {
		$id = $rs["id"];
		
		$pickup_start_date = $rs["pickup_start_date"]; 
		
		$date = '';
		$chk_total = '';
		
		if ($chk_date <> $pickup_start_date) {
			$date = $pickup_start_date;
			
			if ($chk_date <> false) {
				
		$chk_total = '<div class="total">'.number_format($chk_sum).'</div>';
	}
	$chk_sum = 0;
}
			
		$tr_class = '';
		if ($level == '2') {
			$tr_class = ' class="tr2"';
		}
		else if ($level2 == '3') {
			$tr_class = ' class="tr3"';
		}
		echo '<tr align="center"'.$tr_class.'>';
		?>
        
        
        
<?php echo '<td height="42">'.$date.'</td>';?>