如何编写实体的LINQ以加入空表 [英] How to write an LINQ for entities for joining empty tables

问题描述

我有两张桌子：

Hi,

I have two tables :

Category(categ) 
Sub Category (scat)

我想对上表进行以下查询。

I want to perform the following query on the above tables.

select a.code, a.name, a.type, b.category as category from categ a left join scat b on a.code=b.code

以上sql在SQL Server上执行时就是结果。

The above sql when executed on SQL Server this is the result.

code     name      type    category
1001     abc       F       NULL
1002     def       F       NULL
1003     ghi       B       NULL  

我为上述查询尝试了这个LINQ：

请注意：scats上下文表是一个空表。

I tried this LINQ for the above query :
Please note : The scats context table is an empty table.

List<categscat> lstcateg = (from categrow in Context.categs.AsEnumerable()
                            join scatrow in Context.scats
                            on categrow.code equals scatrow.code                             
                            select new categscat(categrow.code, categrow.name, 
                            categrow.type, categrow.cate, 
                            joinedrow.category)).ToList();

返回NullReferenceException。



我尝试过：

<根据以下链接中的建议尝试使用DefaultIfEmpty（）：$ / b
https://www.codeproject.com/ articles / 169590 / linq-to-sql-left-join-with-null-values

This returns a NullReferenceException.

What I have tried:

tried using the DefaultIfEmpty() as per suggestions in the link below :

https://www.codeproject.com/articles/169590/linq-to-sql-left-join-with-null-values

lstcateg = (from categrow in dlsmain.odlsContext.categs.AsEnumerable()
                            join scatrow in dlsmain.odlsContext.scats.AsEnumerable()
                            on categrow.code equals scatrow.code into jointable
                            from joinedrow in jointable.DefaultIfEmpty()
                            select new categscat(categrow.code, categrow.name, categrow.type, categrow.cate, joinedrow.category)).ToList();

我仍​​然得到同样的错误。我的错是什么？如何做到这一点？请建议。

Still I'm getting the same error. What is my mistake? How to get this right ? Please suggest.

推荐答案

修改查询以包含空检查，这很好。



推荐： C＃中的101个LINQ样本 [ ^ ]



修改后的LINQ查询突出显示更改：

Revised the query to include a null check and it was fine.

Referred : 101 LINQ Samples in C#[^]

Revised LINQ query with the change highlighted :

lstcateg = (from categrow in dlsmain.odlsContext.categs.AsEnumerable()
                           join scatrow in dlsmain.odlsContext.scats.AsEnumerable()
                           on categrow.code equals scatrow.code into jointable
                           from joinedrow in jointable.DefaultIfEmpty()
                           select new categscat(categrow.code, categrow.name, categrow.type, categrow.cate, joinedrow == null ? "" : joinedrow.category)).ToList();

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