LINQ:结合加入和分组依据 [英] LINQ: combining join and group by

问题描述

我有一个结合了连接和组的查询，但我遇到了问题.查询如下:

I have a query that combines a join and a group, but I have a problem. The query is like:

 var result = from p in Products                         
 join bp in BaseProducts on p.BaseProductId equals bp.Id                    
 group p by p.SomeId into pg                         
 select new ProductPriceMinMax { 
       SomeId = pg.FirstOrDefault().SomeId, 
       CountryCode = pg.FirstOrDefault().CountryCode, 
       MinPrice = pg.Min(m => m.Price), 
       MaxPrice = pg.Max(m => m.Price),
       BaseProductName = bp.Name  <------ can't use bp. 
 };

如您所见，它将Products 表与BaseProducts 表连接起来，并根据Product 表的id 进行分组.但是在结果ProductPriceMinMax中，我还需要BaseProducts表的一个属性:bp.Name，但它不知道bp.

As you see, it joins the Products table with the BaseProducts table, and groups on an id of the Product table. But in the resulting ProductPriceMinMax, I also need a property of the BaseProducts table: bp.Name, but it doesn't know bp.

知道我做错了什么吗?

推荐答案

完成此操作后

group p by p.SomeId into pg  

您不再可以访问初始 from 中使用的范围变量.也就是不能再讲p或bp，只能讲pg.

you no longer have access to the range variables used in the initial from. That is, you can no longer talk about p or bp, you can only talk about pg.

现在，pg 是一个组，因此包含多个产品.给定 pg 组中的所有产品都具有相同的 SomeId(因为这是您分组的内容)，但我不知道这是否意味着它们都具有相同的 <代码>BaseProductId.

Now, pg is a group and so contains more than one product. All the products in a given pg group have the same SomeId (since that's what you grouped by), but I don't know if that means they all have the same BaseProductId.

要获得基本产品名称，您必须在 pg 组中选择一个特定的产品(就像您使用 SomeId 和 CountryCode>)，然后然后加入BaseProducts.

To get a base product name, you have to pick a particular product in the pg group (As you are doing with SomeId and CountryCode), and then join to BaseProducts.

var result = from p in Products                         
 group p by p.SomeId into pg                         
 // join *after* group
 join bp in BaseProducts on pg.FirstOrDefault().BaseProductId equals bp.Id         
 select new ProductPriceMinMax { 
       SomeId = pg.FirstOrDefault().SomeId, 
       CountryCode = pg.FirstOrDefault().CountryCode, 
       MinPrice = pg.Min(m => m.Price), 
       MaxPrice = pg.Max(m => m.Price),
       BaseProductName = bp.Name  // now there is a 'bp' in scope
 };

也就是说，这看起来很不寻常，我认为您应该退后一步，考虑一下您实际尝试检索的内容.

That said, this looks pretty unusual and I think you should step back and consider what you are actually trying to retrieve.

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