求一付款记录 实现方法！
例如：
总额 已付 未付
50 20 30
50 10 20
50 5 15
这个怎么实现，其中总额和已付分别是两个表里的字段，未付是添加的一个自定义字段，请问该怎么实现，我是用linq查出的数据，主要是方法不知道怎么实现，急求。。。。

[解决办法]
思路：
var query=from t1 in table1
join t2 in table2
on t1.id equals t2.t1Id
select new{总额=t1.总额 ,已付=t2.已付,未付=t1.总额-t2.已付};
[解决办法]
可不可以这样理解：假设一张订单Order，编号为order001，总额是80元，第一次付款20元，第二次付款40元，第3次付款10元，目前还余10元。

然后对应2张表。表一：订单 orders

Assembly code

订单编号    总价order001    80
[解决办法]
作为一个LINQ的新手，这样的题目很有挑战性。想了一个下午，终于想出来了，还请各位LINQ的高手进一步简化和改良，以进一步提高效率，消除冗余。

附上完整的测试代码：
C# code
using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Collections;namespace PaymentRecord{    class Program    {        static void Main(string[] args)        {            ArrayList orders = new ArrayList();            orders.Add(new Order { _id = 1, _total = 80M });            orders.Add(new Order { _id = 2, _total = 100M });            orders.Add(new Order { _id = 3, _total = 20.5M });            ArrayList payments = new ArrayList();            payments.Add(new Payment { _id = 1, _payment = 20M, _date = DateTime.Parse("2011/06/02") });            payments.Add(new Payment { _id = 1, _payment = 40M, _date = DateTime.Parse("2011/06/04") });            payments.Add(new Payment { _id = 1, _payment = 10M, _date = DateTime.Parse("2011/06/12") });            payments.Add(new Payment { _id = 2, _payment = 30M, _date = DateTime.Parse("2011/06/08") });            var payrecords_1 = orders.OfType<Order>()                .GroupJoin(payments.OfType<Payment>(),                o => o._id,                p => p._id,                (o, ps) => ps.DefaultIfEmpty().Select(p => new                {                    id = o._id,                    total = o._total,                    payment = (p != null) ? p._payment : 0,                    paydate = (p != null) ? p._date : (new DateTime(0))                }))                    .SelectMany(r => r);            var payrecords_2 = payrecords_1.Select(s => new            {                id = s.id,                total = s.total,                payment = s.payment,                remain = s.total - payrecords_1.Where(p => (p.id == s.id) && (p.paydate <= s.paydate))                .Sum(sum => sum.payment),                paydate = s.paydate            });            foreach (var item in payrecords_2)                Console.WriteLine(item);        }        class Order { public int _id; public decimal _total; }        class Payment { public int _id; public decimal _payment; public DateTime _date; }    }}