Question

我有以下代码片段，其中我尝试使用左连接。如果没有找到记录，我想将其显示为“无”。

  var customers = new Customer[]
    {
        new Customer{Code = 5, Name = "Sam"},
        new Customer{Code = 6, Name = "Dave"},
        new Customer{Code = 7, Name = "Julia"},
        new Customer{Code = 8, Name = "Sue"}
    };

    // Example orders.
    var orders = new Order[]
    {
        new Order{KeyCode = 5, Product = "Book"},
        new Order{KeyCode = 6, Product = "Game"},
        new Order{KeyCode = 7, Product = "Computer"},
        new Order{KeyCode = 7, Product = "Mouse"},
        new Order{KeyCode = 8, Product = "Shirt"},
        new Order{KeyCode = 5, Product = "Underwear"}
    };

 var source = customers.GroupJoin(
    orders,
    p => p.Code,
    c => c.KeyCode,
    (p, g) => g
        .Select(c => new { PID = p.Code, CID = c.KeyCode })
        .DefaultIfEmpty(new { PID = 0, CID }))//Error   1   The type arguments for method 'System.Linq.Enumerable.DefaultIfEmpty<TSource>(System.Collections.Generic.IEnumerable<TSource>, TSource)' cannot be inferred from the usage. Try specifying the type arguments explicitly.   

    .SelectMany(g => g);
    // Enumerate results.

我了解如何应用群组加入。但是如果我想通过修改上一个查询来显示左外连接记录，需要做哪些更改？

有人请帮助我理解这一点。分配和变量声明如何发生在这里？

编辑：我修改了，因为我找到了另一篇文章，但在行defaultifempty中收到错误。我是否需要修改任何内容才能使其正常工作？

left outer join in lambda/method syntax in Linq

Answer 1

我不确定你的例子是什么是X，Y但是你所寻求的答案要简单得多。

这将加入您的2个表，并使用func（订单，客户），您可以选择结果中的哪些参数，或选择整个对象：

       var query = orders.Join         //Join source table
            (customers,                 //Outer table
            m => m.KeyCode,             //The foreign key from source to outer table
            o => o.Code,                //The 'primary' key of target table
            (order, customer) =>       //func for parameters

            new { order, customer }).GroupBy(m=>m.customer.Code); //Your Result view

            //In sql this is something like:
        /*  SELECT left.Product, 
         *         right.Name 
         *         from Orders as 'left'
         *         left join customers as 'right' on 
         *         left.KeyCode == right.Code
         */

        foreach (var outerItem in query)
        {
            Debug.WriteLine("{0} bought...", outerItem.FirstOrDefault().customer.Name);
            foreach (var item in outerItem)
            {
                Debug.WriteLine("Product {0}", item.order.Product);
            }

        }

group join -left outer join-无法从用法推断出方法'System.Linq.Enumerable.DefaultIfEmpty的类型参数。错误

1 个答案: