如何为涉及自连接的多个连接编写LINQ? [英] How to write LINQ for multiple joins involving self join?

问题描述

我正在尝试通过自我联接将以下查询转换为LINQ表达式.

I am trying to translate the following query with self join to a LINQ expression.

select r2.* from depends d
  join request r on d.DESC =r.DESC
  join request r2 on d.ID=r2.ID
  and d.TYPE ='sometype'
where r.ID= 12345

如何纠正下面的LINQ查询以匹配上面的正确SQL查询?

How can I correct the following LINQ query to match the correct SQL query above?

var result = (from d in depends
            join r in request on d.DESC equals r.DESC
            join r2 in request on d.ID == r2.ID && d.TYPE == incomingType.ToString()
            where r.ID == incomingId
            select r2).AsEnumerable();

推荐答案

如果您的incomingType是固定的，为什么不简单使用Where子句呢?

Why don't you simple use a Where clause if your incomingType is fixed?

还有多种联接条件的语法，如下所示: LINQ在C#中联接多个条件

Also there is syntax for multiple join conditions, as shown here: LINQ Joining in C# with multiple conditions

例如:

var result = (from d in depends
              where d.TYPE == incomingType.ToString()
              join r in request on
              new { ID = r.ID, desc = r.DESC }
              equals 
              new { ID = d.ID, desc = d.DESC }
              where r.ID == incomingId
              select r).AsEnumerable();

如果incomingType实际上没有固定，并且它从depends表中获取其值，则只需在连接条件中添加第三个参数即可，例如

If your incomingType is not actually fixed, and it gets its value from the depends table, you can just add a third parameter to the join condition, e.g.

var result = (from d in depends
              where d.TYPE == incomingType.ToString()
              join r in request on
              new { ID = r.ID, desc = r.DESC, type = r.someType1 }
              equals 
              new { ID = d.ID, desc = d.DESC, type = d.someType2 }
              where r.ID == incomingId
              select r).AsEnumerable();

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