您如何在LINQ中找到按组的最大值? [英] How do you find the group-wise max in LINQ?

问题描述

我正在尝试解决LINQ中的按组最大"问题.首先，我有一个使用实体框架建模的数据库，其结构如下:

I'm trying to solve the "group-wise max" problem in LINQ. To start, I have a database modeled using the Entity Framework with the following structure:

Customer:
---------
CustomerID : Int32
Name : String

Order:
-------
OrderID : Int32
CustomerID : Int32
Total : Decimal

这使我可以从客户导航到她的订单，并从订单导航到所有者.

This gives me navigation from a Customer to her orders and an Order to the owner.

我正在尝试创建一个LINQ查询，该查询允许我在数据库中查找前10名客户订单.简单的案例很容易想到:

I'm trying to create a LINQ query that allows me to find the top-10 customer orders in the database. The simple case was pretty easy to come up with:

var q = (
    from order in _data.Orders  // ObjectQuery<Order>
    orderby order.Amount descending select order
).Take(10);

但是，我只想在此列表中显示唯一客户.我对LINQ还是有点陌生​​，但这是我想出的:

However, I'd like to only show unique customers in this list. I'm still a bit new to LINQ, but this is what I've come up with:

var q = (
    from order in _data.Orders  // ObjectQuery<Order>
    group order by order.Customer into o
    select new {
        Name = o.Key.Name,
        Amount = o.FirstOrDefault().Amount
    }
).OrderByDescending(o => o.Amount).Take(10);

这似乎可行，但是我不确定这是否是最佳方法.具体来说，我想知道针对大型数据库进行此类查询的性能.另外，使用组查询中的FirstOrDefault方法看起来有些奇怪...

This seems to work, but I'm not sure if this is the best approach. Specifically, I wonder about the performance of such a query against a very large database. Also, using the FirstOrDefault method from the group query looks a little strange...

任何人都可以提供更好的方法，或者可以保证这是正确的方法吗?

Can anyone provide a better approach, or some assurance that this is the right one?

推荐答案

您可以这样做:

var q = (
    from order in _data.Orders  // ObjectQuery<Order>
    orderby order.Amount descending select order
).Distinct().Take(10);

我通常会查看生成的SQL，然后看看什么是最好的.

I would normally look at the generated SQL, and see what is the best.

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