Mysql 数据库有关问题，该语句是什么意思

$query = "select orderid from orders where  <br />               customerid = $customerid and  <br />               amount > ".$_SESSION['total_price']."-.001 and //这句是什么意思？ <br />               amount < ".$_SESSION['total_price']."+.001 and  //这句是什么意思？ <br />               date = '$date' and <br />               order_status = 'PARTIAL' and <br />               ship_name = '$ship_name' and <br />               ship_address = '$ship_address' and <br />               ship_city = '$ship_city' and <br />               ship_state = '$ship_state' and <br />               ship_zip = '$ship_zip' and <br />               ship_country = '$ship_country'";