如何在 2 个谓词上对 2 个表进行 LINQ 左连接？

Posted 2023-02-24

【中文标题】如何在 2 个谓词上对 2 个表进行 LINQ 左连接？

【英文标题】：How to do LINQ left joins on 2 predicates to 2 tables?

【发布时间】：2021-05-15 10:22:25

【问题描述】：

SELECT *
FROM A
LEFT JOIN B ON A.HelloID = B.HelloID 
LEFT JOIN C ON A.BusyID = C.BusyID AND B.MemID = C.MemID 

我尝试了以下方法：

var list = from A in this.A
           join B in this.B
              on A.HelloID equals B.HelloID into bgj
           from subb in bgj.DefaultIfEmpty()
           join C in this.C
              on B.MemID equals C.MemID into cgj
           from subc in cgj.DefaultIfEmpty()
              on A.BusyID equals C.BusyID into cgj2
           from subc2 in cgj2.DefaultIfEmpty()

但这不起作用，有什么想法吗？

【问题讨论】：

一方面，应该是.DefaultIfEmpty() 而不是.DefaultIfempty

好电话！让我补充一下。

【参考方案1】：

试试这个：

var list = from A in this.A
           join B in this.B
              on A.HelloID equals B.HelloID into bgj
           from B in bgj.DefaultIfEmpty()
           join C in this.C
              on A.BusyId equals C.BusyId into cgj
           from C in cgj.DefaultIfEmpty()
           where B.MemID == C.MemID 

您应该澄清有关 MemID 和 BusyId 的问题。如果它们可以为空。在这种情况下，最后一行可能是这样的：

where (B.MemID==null || B.MemID == C.MemID )

//Or 

where ((B.MemID==null && C.MemID==null)  || (B.MemID == C.MemID ))

【讨论】：

这种导致C.MemID是强制的，变成了inner join而不是left join？

【参考方案2】：

var list = from A in this.A
           join B in this.B
              on A.HelloID equals B.HelloID into bgj
           from subb in bgj.DefaultIfEmpty()
           join C in this.C
             on new  colA = A.BusyID, colB = B.MemID   equals new  colA = C.BusyID, colB = C.MemID  into cgj
           from subc in cgj.DefaultIfEmpty()