LINQ to SQL:在多个列上进行多个连接.这可能吗？

Question

鉴于:

以TABLE_1下列列命名的表:

• ID
• ColumnA
• ColumnB
• ColumnC

我有SQL查询,其中TABLE_1对自己加入基于两次关闭的ColumnA,ColumnB,ColumnC.查询可能如下所示:

Select t1.ID, t2.ID, t3.ID
  From TABLE_1 t1
  Left Join TABLE_1 t2 On
       t1.ColumnA = t2.ColumnA
   And t1.ColumnB = t2.ColumnB
   And t1.ColumnC = t2.ColumnC
  Left Join TABLE_1 t3 On
       t2.ColumnA = t3.ColumnA
   And t2.ColumnB = t3.ColumnB
   And t2.ColumnC = t3.ColumnC
... and query continues on etc.

问题:

我需要在LINQ中重写Query.我试过去刺它:

var query =
    from t1 in myTABLE1List // List<TABLE_1>
    join t2 in myTABLE1List
      on t1.ColumnA equals t2.ColumnA
      && t1.ColumnB equals t2.ColumnA
    // ... and at this point intellisense is making it very obvious
    // I am doing something wrong :(

如何在LINQ中编写查询？我究竟做错了什么？

Answer 1

加入Linq to SQL中的多个列有点不同.

var query =
    from t1 in myTABLE1List // List<TABLE_1>
    join t2 in myTABLE1List
      on new { t1.ColumnA, t1.ColumnB } equals new { t2.ColumnA, t2.ColumnB }
    ...

您必须利用匿名类型并为要比较的多个列组成一个类型.

这一开始似乎令人困惑,但是一旦你熟悉了SQL从表达式组成的方式它会更有意义,在封面下这将生成你正在寻找的连接类型.

编辑根据注释添加第二个连接的示例.

var query =
    from t1 in myTABLE1List // List<TABLE_1>
    join t2 in myTABLE1List
      on new { A = t1.ColumnA, B = t1.ColumnB } equals new { A = t2.ColumnA, B = t2.ColumnB }
    join t3 in myTABLE1List
      on new { A = t2.ColumnA, B =  t2.ColumnB } equals new { A = t3.ColumnA, B = t3.ColumnB }
    ...

Answer 2

在LINQ2SQL中,在使用内部联接时很少需要显式连接.

如果数据库中有正确的外键关系,您将自动在LINQ设计器中获得关系(如果不是,您可以在设计器中手动创建关系,尽管您应该在数据库中确实存在正确的关系)

然后您可以使用"点符号"访问相关表

var q = from child in context.Childs
        where child.Parent.col2 == 4
        select new
        {
            childCol1 = child.col1,
            parentCol1 = child.Parent.col1,
        };

将生成查询

SELECT [t0].[col1] AS [childCol1], [t1].[col1] AS [parentCol1]
FROM [dbo].[Child] AS [t0]
INNER JOIN [dbo].[Parent] AS [t1] ON ([t1].[col1] = [t0].[col1]) AND ([t1].[col2] = [t0].[col2])
WHERE [t1].[col2] = @p0
-- @p0: Input Int (Size = -1; Prec = 0; Scale = 0) [4]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 4.0.30319.1

在我看来,这更具可读性,让你专注于你的特殊条件而不是连接的实际机制.

编辑
当然,只有当您想要加入我们的数据库模型时才适用.如果你想加入"模型之外"你需要求助于手动加入作为答案从昆廷·罗宾逊

Answer 3

Title_Authors是一个查找两件事情,一次加入项目结果并继续链接

        DataClasses1DataContext db = new DataClasses1DataContext();
        var queryresults = from a in db.Authors                                          
                    join ba in db.Title_Authors                           
                    on a.Au_ID equals ba.Au_ID into idAuthor
                    from c in idAuthor
                    join t in db.Titles  
                    on c.ISBN equals t.ISBN 
                    select new { Author = a.Author1,Title= t.Title1 };

        foreach (var item in queryresults)
        {
            MessageBox.Show(item.Author);
            MessageBox.Show(item.Title);
            return;
        }

Answer 4

你还可以使用:

var query =
    from t1 in myTABLE1List 
    join t2 in myTABLE1List
      on new { ColA=t1.ColumnA, ColB=t1.ColumnB } equals new { ColA=t2.ColumnA, ColB=t2.ColumnB }
    join t3 in myTABLE1List
      on new {ColC=t2.ColumnA, ColD=t2.ColumnB } equals new { ColC=t3.ColumnA, ColD=t3.ColumnB }