如何做好左联接在LINQ在单个连接多个字段 [英] How to do left joins in LINQ on multiple fields in single join

问题描述

我试图做到这一点简单的SQL查询LINQ。但它给我的错误。

下面是需要CONVER到LINQ SQL查询

 定义@groupID INT 
设置@groupID = 2 
选择* 
 FROM dbo.Person p 
 LEFT JOIN dbo.PersonGroup皮克ON（p.PersonID = pg.PersonID和pg.GroupID = @groupID）
  

忽略@groupID。这将提供为LINQ查询功能的参数。

下面是LINQ查询什么，我都试过了。

<$ P在消费
$ p> 从对加入皮克在PersonGroup新{p.PersonID，GROUPID}等于新的{pg.PersonID，pg.GroupID}成T
从RT在t.DefaultIfEmpty（）


在哪里GROUPID作为函数参数提供。这两个groupId和是PersonID是int类型。但它给了我下面的错误，

 错误2联接子句中的类型表现之一是不正确。类型推断在调用群组加入失败。 
  

小的帮助，将不胜感激。

解决方案

您代码

 从人物
 p加入皮克在PersonGroup新{p.PersonID，GROUPID}在t.DefaultIfEmpty（）等于新的{pg.PersonID，pg.GroupID}成T 
从RT 
  

从p改变为

 在人物
在PersonGroup加入皮克新{=人p.PersonID，组= GROUPID}等于新{=人pg.PersonID，组= pg.GroupID}入T 
从RT在t.DefaultIfEmpty（）
  

这样，它会加入使用匿名类型

I am trying to do this simple sql query to LINQ. But its give me error.

Here is the SQL query that need to conver to LINQ

 DECLARE @groupID int
 SET @groupID = 2
 SELECT * 
    FROM dbo.Person p
    LEFT JOIN dbo.PersonGroup pg ON ( p.PersonID = pg.PersonID AND pg.GroupID = @groupID)

Ignore @groupID. which will be provided as function parameter for LINQ query.

Here is LINQ query what i have tried.

from p in Person
 join pg in PersonGroup on new { p.PersonID, groupID } equals new { pg.PersonID, pg.GroupID } into t
 from rt in t.DefaultIfEmpty()

Where groupID is provided as function parameter. Both GroupID and PersonID are int. But it gives me following error,

Error   2   The type of one of the expressions in the join clause is incorrect.  Type inference failed in the call to 'GroupJoin'.

Little help would be appreciated.

解决方案

Your Code

from p in Person
 join pg in PersonGroup on new { p.PersonID, groupID } equals new { pg.PersonID, pg.GroupID } into t
 from rt in t.DefaultIfEmpty()

Change it to

from p in Person
 join pg in PersonGroup on new { Person = p.PersonID, Group = groupID } equals new { Person = pg.PersonID, Group = pg.GroupID } into t
 from rt in t.DefaultIfEmpty()

That way it will join using the Anonymous type

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