左Linq中加入吗？ [英] Left join in Linq?

问题描述

有很多关于SO已经有大约左Linq中的联接问题，我看了所有的那些使用加入关键字达到理想的终点。

There are a lot of questions on SO already about Left joins in Linq, and the ones I've looked at all use the join keyword to achieve the desired end.

这是没有道理给我。比方说，我有表客户和发票，由外键联系客户ID 在发票。现在，我想运行一个包含客户信息的报告，以及任何发票。 SQL是：

This does not make sense to me. Let's say I have the tables Customer and Invoice, linked by a foreign key CustomerID on Invoice. Now I want to run a report containing customer info, plus any invoices. SQL would be:

select c.*, i.*
  from Customer c
  left join Invoice i on c.ID = i.CustomerID

这是我所看到的答案在SO，人们大多提示：

From what I've seen of the answers on SO, people are mostly suggesting:

var q = from c in Customers
        join i in Invoices.DefaultIfEmpty() on c.ID equals i.CustomerID 
        select new { c, i };

我真不明白这是如何的唯一途径。 客户之间的关系和发票已经由LinqToSQL类中定义;为什么我应该重复它为加入条款？如果我想要一个内部联接它只会一直：

I really don't understand how this can be the only way. The relationship between Customer and Invoice is already defined by the LinqToSQL classes; why should I have to repeat it for the join clause? If I wanted an inner join it would simply have been:

var q = from c in Customers
        from i in c.Invoices
        select new { c, i };

在不指定加盟领域！

without specifying the joined fields!

我想：

var q = from c in Customers
        from i in c.Invoices.DefaultIfEmpty()
        select new { c, i };

但只是给了我同样的结果，好像它是一个内连接。

but that just gave me the same result as if it were an inner join.

请问有没有这样做的更好的办法？

Is there not a better way of doing this?

推荐答案

你在说什么？这从我c.Invoice.DefaultIfEmpty（）正是左连接。

What are you talking about? That from i in c.Invoice.DefaultIfEmpty() is exactly a left join.

        List<string> strings = new List<string>() { "Foo", "" };

        var q = from s in strings
                from c in s.DefaultIfEmpty()
                select new { s, c };

        foreach (var x in q)
        {
            Console.WriteLine("ValueOfStringIs|{0}| ValueOfCharIs|{1}|",
                x.s,
                (int)x.c);
        }

本测试将生成：

ValueOfStringIs|Foo| ValueOfCharIs|70|
ValueOfStringIs|Foo| ValueOfCharIs|111|
ValueOfStringIs|Foo| ValueOfCharIs|111|
ValueOfStringIs|| ValueOfCharIs|0|

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