请指导，简单的数据库语句

请指点，简单的数据库语句
$query = "select orderid from orders where
customerid = $customerid and
amount > ".$_SESSION['total_price']."-.001 and //这句是什么意思？ ."-.001 是什么意思？
amount < ".$_SESSION['total_price']."+.001 and //这句是什么意思？ ."+.002是什么意思？
date = '$date' and
order_status = 'PARTIAL' and
ship_name = '$ship_name' and
ship_address = '$ship_address' and
ship_city = '$ship_city' and
ship_state = '$ship_state' and
ship_zip = '$ship_zip' and
ship_country = '$ship_country'";
------解决方案--------------------
-0.001
+0.001
------解决方案--------------------
total_price 取值上下浮动 0.001饭碗内

例如
total_price = 1.001
取值 1.000 到 1.002
------解决方案--------------------
就是算术运算的-0.001 与 +0.001

例如$_SESSION['total_price'] 的值是 1

那么语句就相当于

$query = "select orderid from orders where
customerid = $customerid and
amount >0.999 and
amount < 1.001 and
date = '$date' and
order_status = 'PARTIAL' and
ship_name = '$ship_name' and
ship_address = '$ship_address' and
ship_city = '$ship_city' and
ship_state = '$ship_state' and
ship_zip = '$ship_zip' and
ship_country = '$ship_country'";