将所有值一起添加到SQL SELECT中

Question

How do I add all the amounts together so that instead of this select script returning all the individual withdrawal amounts return one number of them all added together?

<?php
$end_date = strtotime($nextdate);
$start_date = strtotime($statement_date);
$username = $usr['username'];
$result = mysql_query("SELECT * FROM `usr_withdraw` WHERE `timestamp` > '$start_date' AND `timestamp` < '$end_date' AND `username` = '$username'")or die(mysql_error());

while($balance = mysql_fetch_array( $result )) { 
    echo $balance['amount'];
}
?>

Answer 1

To return only 1 row with the aggregate sum of all amounts use the sql sum() function.

SELECT sum(amount) FROM `usr_withdraw` WHERE ...