如何获得表列的总和？

I have a column name quantity, unit_price and total_amount. The total_amount field in my database set to zero, in my query I used SUM(unit_price*quantity) AS total_amount to get the total value as total_amount. But when I tried to get the sum of total amount as total_sum is always get 0. if is it because the total_amount field in my database is set to zero? What I want to do is get the SUM(total_amount) AS total_sum but it always displays 0.

This is my Query.

<?php
$mysqli = new mysqli("localhost", "root", "", "app");

    $term = $_GET['supp'];
    $result = $mysqli->query("SELECT *, SUM(unit_cost*quantity) AS total_amount FROM procurement WHERE supplier LIKE '%".$term."%' GROUP BY counter");
    echo'<table id="tfhover" cellspacing="0" class="tablesorter" style="text-transform:uppercase;" border="1px">
        <thead>
        <tr>
    <th>SUPPLIER</th>
    <th>ITEM</th>
    <th>DESCRIPTION</th>
    <th>QTY</th>
    <th>UNIT</th>
    <th>UNIT PRICE</th>
    <th>Total Amount</th>
        </tr>
        </thead>';
        echo'<tbody>';
while($row = $result->fetch_assoc()){
 echo'<tr>
            <td>'.$row['supplier'].'</td>
            <td>'.$row['item_name'].'</td>
            <td>'.$row['item_description'].'</td>
            <td>'.$row['quantity'].'</td>
            <td>'.$row['unit'].'</td>
            <td>'.number_format($row['unit_cost'], 2, '.', ',').'</td>
            <td>'.number_format($row['total_amount'], 2, '.', ',').'</td>
       </tr>';
       }
       echo'<TR> <TD COLSPAN=6 BGCOLOR="#99CCFF">TOTAL AMOUNT</TD> <td>'.number_format($row['total_sum'], 2, '.', ',').'</td></TR>';
    echo "</tbody></table>";

?>

My another problem is when I insert this code echo' TOTAL AMOUNT '.number_format($row['total_sum'], 2, '.', ',').''; inside the while loop. I want to have just 1 total amount row.

SELECT *, SUM(unit_cost*quantity) AS total_amount FROM procurement WHERE supplier LIKE '%".$term."%' GROUP BY counter"

This is a select statement which would not update any data in the database. It seems that your total_amount, which can be evaluated every time when you need it, is not necessarily exists as a column in your table.

Also, can you post your table structure ? Actually the SQL seems not correct anyway.