C# LINQ to Entities group by 和 Count()
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LINQ to Entities group by and Count()
提问by Jim S
I have the following LINQ-to-Entities query
我有以下 LINQ-to-Entities 查询
from r in ctx.Rs
join p in ctx.Ps on r.RK equals p.RK
group r by r.QK into gr
select new { QK = (int)gr.Key, Num = gr.Count() }
that runs against this schema
针对此架构运行的
Table P Table R Table Q
PK*
RK ----> RK*
Text QK ------> QK*
Text Text
and gives this message if there is any record in Q with no corresponding record in P: "The cast to value type 'Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type."
并在 Q 中有任何记录而 P 中没有相应记录时给出此消息：“转换为值类型 'Int32' 失败，因为具体化值为空。结果类型的泛型参数或查询必须使用可空类型。 ”
The problem is the gr.Count() in the last line, but I cannot find a solution. I have tried to test gr for null, but cannot find a way that works.
问题是最后一行的 gr.Count()，但我找不到解决方案。我试图测试 gr 是否为 null，但找不到有效的方法。
I have seen a number of solutions to a similar problem using Sum() instead of Count(), but I have not been able to adapt them to my problem.
我已经看到许多使用 Sum() 而不是 Count() 来解决类似问题的解决方案，但我无法使它们适应我的问题。
I tried changing my query to look like the one in Group and Count in Linq issue, but I just got a different message.
我尝试将我的查询更改为类似于Linq 问题中Group 和 Count 中的查询，但我收到了不同的消息。
I also looked at Group and Count in Entity Framework(and a number of others) but the problem is different.
我还查看了实体框架（以及其他一些框架）中的Group 和 Count，但问题有所不同。
回答by Joe
I'm having trouble reading your format. But can you try:
我在阅读您的格式时遇到问题。但是你能不能试试：
from r in ctx.Rs
join p in ctx.Ps.DefaultIfEmpty() on r.RK equals p.RK
group r by r.QK into gr
select new { QK = (int)gr.Key, Num = gr.Count(x => x.RK != null) }
With DefaultIfEmptyand x => x.RK != nullbeing the changes.
随着DefaultIfEmpty和x => x.RK != null正在发生变化。
回答by aifarfa
group Key can't be null
组键不能为空
var results = ctx.Rs.Where(r => r.QK != null)
.GroupBy(r => r.QK)
.Select(gr => new { Key = (int)gr.Key, Count = gr.Count() }
.ToList();
PS.
附注。
Mostly, You don't need 'JOIN' syntax in Entity Framework. see: Loading Related Entities
Writing descriptive-meaningful variable names would significantly improve Your codes and make it understandable. Readability does matter in real world production.
大多数情况下，您不需要实体框架中的“JOIN”语法。请参阅：加载相关实体
编写具有描述性意义的变量名称将显着改进您的代码并使其易于理解。可读性在现实世界的生产中很重要。
