C# 如何从复杂的SQL创建LINQ？

我有以下SQL查询：

 SELECT D.ID
 FROM Documents AS D
 INNER JOIN DocClasses AS DC WITH (NOLOCK) 
 ON D.DocClass = DC.ID
 INNER JOIN DocSubClasses AS DSC WITH (NOLOCK) 
 ON D.DocSubClass = DSC.ID AND DSC.DocClassID = DC.ID
 INNER JOIN DocPathFolders AS F WITH (NOLOCK) 
 ON D.DocPathFolderID = F.ID
 WHERE 
    DC.ShortName = 'PAY' AND DSC.Name = 'xxxxx' 
    AND UPPER(F.Description) = 'READY TO SEND'

我正在尝试将此查询转换为LINQ。以下是我迄今为止所做的工作：

 from D in ctx.Documents
 join DC in ctx.DocClasses on D.DocClass equals DC.ID
 join DSC in ctx.DocSubClasses 
     on new { D.DocSubClass, DSC.DocClassID } equals new { DSC.ID, DC.ID }
 join F in ctx.DocPathFolders 
     on D.DocPathFolderID equals F.ID
 where 
     DC.ShortName == "PAY" 
     && DSC.Name == "xxxxx" 
     && (F.Description).ToUpper() == "READY TO SEND"
 select D.ID;

我在这行中遇到了错误：

join DSC in ctx.DocSubClasses on new { D.DocSubClass, DSC.DocClassID } equals new { DSC.ID, DC.ID }

在这里，我得到以下错误：

名称“DSC”不在“equals”左侧的范围内

名称“DC”不在“equals”右侧的范围内

---

我是LINQ的新手，这就是为什么我不能解决这些错误。我愿意听取关于上述问题的任何建议。谢谢。

以下是我解决问题的方法：

from D in ctx.Documents
join DC in ctx.DocClasses on D.DocClass equals DC.ID
join DSC in ctx.DocSubClasses on new { D.DocSubClass, DC.ID } equals new { DocSubClass = DSC.ID, ID = DSC.DocClassID }
join F in ctx.DocPathFolders on D.DocPathFolderID equals F.ID
where DC.ShortName == "PAY" && DSC.Name == "xxxx" && (F.Description).ToUpper() == "READY TO SEND"
select D.ID;

---

您应该重新排列匿名对象中的属性，如下所示：

join DSC in ctx.DocSubClasses 
on new { D.DocSubClass, DC.ID } equals new { DSC.DocClassID, DSC.ID }

---

右边的应该是您要加入的表（

DSC

），左边的可以是以前的表

只要将DSC切换到equals的右边，将DC切换到左边，它就可以正常工作了

join DSC in ctx.DocSubClasses on new { D.DocSubClass, DC.ID } equals new { DSC.ID, DSC.DocClassID}

---

提示：如果您在数据库中正确设置了关系，您几乎不需要使用“join”。用LINQ编写更容易，关系作为“导航属性”公开。您只需使用“点符号”。即：

注意：我们对您的型号没有任何想法。我假设这种关系是一对多的。如果是多对1，那么您只需使用如下符号：

where d.DocClass.ShortName == "PAY" 
                  && d.DocClass.DocSubClass.Name == "xxxxx" 
                  && d.DocPathFolder.Description.ToUpper() == "READY TO SEND" 

---

HINT2：从Linqpad.net下载并开始使用Linqpad。这是一个很好的工具，您不仅可以测试LINQ查询，还可以将其用作.Net记事本。

您可以尝试使用

Lambda

而不是

LINQ

在

Lambda

中，您的

JOIN

语句如下所示

#

---

比较错误的值，因为

ID

和

DocSubClass

未与相同的属性名称进行比较。您必须更改任一匿名类中的属性名称（将其切换）

where d.DocClass.ShortName == "PAY" 
                  && d.DocClass.DocSubClass.Name == "xxxxx" 
                  && d.DocPathFolder.Description.ToUpper() == "READY TO SEND" 

var result = ctx.Documents.Join(ctx.DocClasses , d => new { Id = d.DocClass }, dc => new { Id = dc.ID }, (d, dc) => new { doc = d, docClass = dc }) //INNER JOIN DocClasses  
                          .Join(ctx.DocSubClasses , d => new { sc = d.doc.DocSubClass, Id = d.docClass.ID }, dsc => new { sc = dsc.ID, Id = dsc.DocClassID }, (d, dsc) => new { doc = d, dsc = dsc } ) //INNER JOIN DocSubClasses
                          .Join(ctx.DocPathFolders, d => new { fId = d.doc.doc.DocPathFolderID }, f => new { fId = f.ID }, (d, f) => new { doc = d, f = f }) //INNER JOIN DocPathFolders
                          .Where(x => x.doc.doc.docClass.ShortName == "PAY" && x.doc.dsc.Name == "xxxxx" && x.f.Description.ToUpper() == "READY TO SEND")//Apply where clause
                          .Select(y => y.doc.doc.doc.ID);//Select whatever you want