LINQ：结合加入和组 [英] LINQ: combining join and group by

问题描述

我有结合连接的查询和一组，但我有一个问题。查询是这样的：

  VAR的结果=从产品
 p加入BaseProducts英国石油公司p.BaseProductId等于基点.ID 
群p由p.SomeId成PG 
选择新ProductPriceMinMax {
 SomeId = pg.FirstOrDefault（）。SomeId，
 COUNTRYCODE = pg.FirstOrDefault（）。COUNTRYCODE， 
 MinPrice = pg.Min（M = GT; m.Price），
 MaxPrice = pg.Max（M = GT; m.Price），
 BaseProductName = bp.Name<  - -----不是可以使用BP。 
}; 
  

正如你所见，它加入与BaseProducts表中的Products表和组上的一个id产品表。但在所得ProductPriceMinMax，我还需要BaseProducts表的属性：bp.Name，但是它不知道碱基。

任何想法，我做错了吗？

谢谢！

< DIV CLASS =h2_lin>解决方案

一旦你做到了这一点。

  p组为p .SomeId成PG 
  

你不再有机会获得在最初的<$ C $使用的范围变量C>从。也就是说，你可以不再谈 P 或 BP ，你只能说说皮克。

现在，皮克为组所以包含的多个的产品。所有的产品在给定的皮克组具有相同的 SomeId （因为这就是你通过分组），但我不知道，如果这意味着他们都具有相同的 BaseProductId 。

要得到一个基本产品的名称，你有挑皮克组中某一特定产品（如你与 SomeId 和 COUNTRYCODE做）和然后的加盟 BaseProducts 。

  VAR的结果=从产品
群p p由p.SomeId成PG 
 //加盟* *后组
在BaseProducts加入英国石油公司pg.FirstOrDefault（）。BaseProductId等于bp.Id 
选择新ProductPriceMinMax {
 SomeId = pg.FirstOrDefault（）。SomeId，
 COUNTRYCODE = pg.FirstOrDefault（）COUNTRYCODE，
 MinPrice = pg.Min（M = GT; m.Price），
 MaxPrice = pg.Max（M = GT; m.Price），
 BaseProductName = bp.Name //现在有范围
}一个'基点'; 
  

这是说，这看起来非常不寻常的，我觉得你应该退一步考虑一下你实际上是试图检索。

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. 
 };

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.

Any idea what I'm doing wrong?

Thanks!

解决方案

Once you've done this

group p by p.SomeId into 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.

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.

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
 };

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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