Linq问题中的分组和计数 [英] Group and Count in Linq issue

问题描述

我正在使用LINQToSQL进行C#项目，但是现在我对以下查询有疑问:

I am working on a C# project using LINQToSQL,But right now I am having a problem with the following query:

var groupByQuery =  (from c in db.Customers
                            join o in db.Orders on c.CustomerID equals o.CustomerID into lf1
                            from or in lf1.DefaultIfEmpty()
                            group c by c.CustomerID into g
                            select new
                            {
                                CustomerID = g.Key,
                                Count = g.Count()
                            }
                            ).OrderBy(x => x.Count);

如您所见，我正在执行LEFT OUTER JOIN并按CustomerID分组，到目前为止一切正常.但是，当我看到结果时，我意识到没有任何Order的Customers在其Count字段中具有"1".

As you can see I am doing a LEFT OUTER JOIN and grouping by the CustomerID, and everything goes well so far. But when I see the results I realize that the Customers that does not have any Order, has a "1" in their Count field.

那是为什么?在这种情况下，Count字段应该为"0"，我在做什么错了?

Why is that? The Count field is supposed to have a "0" in these cases, What am I doing wrong?

我在这里发现了以下问题:

I found this questions here:

linq-count-query-returns-a -1-而不是a-0

linq-to-sql-joining-query -returning-1-inst-of-0

但是他们都没有对我有帮助，我希望有人能帮助您，谢谢您.

but none of them has been helpful to me, I hope someone can help, thank you in advance.

推荐答案

您使事情变得太复杂了. join...into执行组联接，因此订单按<在您的前两行代码中.这应该起作用:

You're making things too complicated. join...into performs a group join, so the Orders are grouped by CustomerId within your first two lines of code. This should work:

var groupByQuery =  (from c in db.Customers
                     join o in db.Orders on c.CustomerID equals o.CustomerID into lf1
                     select new
                     {
                         CustomerID = c.Id,
                         Count = lf1.Count()
                     }
                     ).OrderBy(x => x.Count);

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