Linq选择最接近日期的记录 [英] Linq select records closest to date

问题描述

如何使用Linq选择最接近指定日期的记录?这用于具有日期，产品ID，位置ID和余额的交易表.

How do you use Linq to select the record that is closest to a specified date? This is for a transaction table that has a date, product id, location id, and balance.

鉴于这些要求:

• 如果指定的日期有几笔交易，请选择当天的最后一笔交易
• 如果指定的日期没有交易，请选择最接近的先前交易
• 在多个位置(例如仓库)显示余额-每个仓库将进行单独的交易
• 显示多种产品的余额-每个产品将有单独的交易

表格数据:

// code
Id, TransDateTime,    ProductId, WarehouseId, Balance
1,  1-Jan-2011 00:00, 1,         1,           100
2,  1-Jan-2011 00:00, 1,         2,           10
3,  2-Jan-2011 00:00, 1,         1,           150
4,  3-Jan-2011 00:00, 1,         2,           25
5,  3-Jan-2011 00:00, 2,         1,           333
6,  7-Jan-2011 00:00, 1,         1,           149
7,  7-Jan-2011 01:00, 1,         2,           30
8,  7-Jan-2011 02:00, 1,         2,           35

测试日期和输出

Date: 1-Jan would output:
1,  1-Jan-2011 00:00, 1,         1,           100
2,  1-Jan-2011 00:00, 1,         2,           10

Date: 3-Jan would output:
3,  2-Jan-2011 00:00, 1,         1,           150 
4,  3-Jan-2011 00:00, 1,         2,           25
5,  3-Jan-2011 00:00, 2,         1,           333
// product 1, warehouse 1 wasn't sold on the 3rd
// so the row from 2-Jan is returned

Date: 7-Jan would output:
5,  3-Jan-2011 00:00, 2,         1,           333
6,  7-Jan-2011 00:00, 1,         1,           149
9,  7-Jan-2011 02:00, 1,         2,           35
// product 2, warehouse 1 wasn't sold on the 7th
// so the row from 3-Jan is returned
// product 1, warehouse 2 was sold twice on the 7th
// so the later one is used

我认为这将需要对分组进行分组(产品->仓库->日期)或类似的分组.这超出了我的能力！

I think it's going to require grouping of groups (product -> warehouse -> date) or similar. Its beyond my linq ability!

推荐答案

步骤:
1)筛选出inputDate
之后发生的交易 2)按产品和仓库对其余交易进行分组
3)在每个组中找到最近的交易
4)格式化结果对象

Steps:
1) Filter out transactions that happened after inputDate
2) Group rest of transactions by product and warehouse
3) In each group find most recent transaction
4) Format result object

直接实现:

DateTime inputDate = ...;
var result = transactions
          .Where(t => t.TransDateTime.Date <= inputDate.Date)
          .GroupBy(t => new {t.ProductId, t.WarehouseId})
          .Select(x => new {
               x.Key,
               LastTransaction = x.OrderByDescending(t => t.TransDateTime).First(),
          })
          .Select(x => new {
               Id = x.LastTransaction.Id,
               Date = x.LastTransaction.TransDateTime,
               ProductId = x.Key.ProductId,
               WarehouseId = x.Key.WarehouseId,
               Balance = x.LastTransaction.Balance,
          });

如果要进行一些优化，可以考虑为IEnumerable实现MaxBy扩展方法来代替x.OrderByDescending(t => t.TransDateTime).First().如果您有很多事务，因为它是O(n)而不是O(n log n)，它将提高性能. MaxBy实现可以在这里使用，例如: C#中的简单LINQ问题

If you want some optimizations you can consider implementing MaxBy extension method for IEnumerable to replace x.OrderByDescending(t => t.TransDateTime).First(). It will improve performance if you have many transactions since it is O(n) instead of O(n log n). MaxBy implementation can be taken here for example: Simple LINQ question in C#

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