请大神给讲解下面mysql语句的作用，谢谢！

请大神给讲解下面mysql语句的作用，谢谢！
`$query = "select orderid from orders where

           customerid = $customerid and  
           amount > ".$_SESSION['total_price']."-.001 and //这句是什么意思？ 
           amount < ".$_SESSION['total_price']."+.001 and  //这句是什么意思？ 
           date = '$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'"; `