问题描述

关于 Linq 中的左连接已经有很多关于 SO 的问题，我看过的问题都使用 join 关键字来实现所需的结果.

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.

这对我来说没有意义.假设我有 Customer 和 Invoice 表，它们由 Invoice 上的外键 CustomerID 链接.现在我想运行一个包含客户信息以及任何发票的报告.SQL 将是:

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

从我所看到的关于 SO 的答案来看，人们大多是在建议:

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 };

我真的不明白这怎么可能是唯一的方法.Customer 和 Invoice 之间的关系已经由 LinqToSQL 类定义；为什么我必须为 join 子句重复它?如果我想要一个内部连接，那就是:

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 };

没有指定连接的字段！

我试过了:

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

但这只是给了我与内部连接相同的结果.

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

没有更好的方法吗?

推荐答案

你在说什么?那个 from i in c.Invoice.DefaultIfEmpty() 正是一个左连接.

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);
        }

这个测试产生:

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

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