Question

在以下LINQ查询中，我需要显示All客户的订单总数，包括未下订单的客户：

模型：

Public class Customer
{
   public int CustomerId { get; set; }
   public string Name{ get; set; }
}

Public class Order
{
   public int OrderId { get; set; }
   public int CustomerId { get; set; }
}

LINQ查询：问题：如何显示ALL客户（包括没有订单的客户）和每位客户的订单总数（对于那些客户ID不在的客户为零）订单表）

var Query1 = from c in Customers
             join o in Orders into co
             from t in co.DefaultIfEmpty()
             select new {CustomerID = c.CustomerId, OrderID = (t == null ? 0 : t.OrderId)};

Answer 1

这不是非常有效，因为它为每个客户迭代Orders，但它可以完成工作：

var query = Customers
    .Select(c => new
        {
            Name = c.Name,
            NumOrders = Orders.Count(o => o.CustomerId = c.CustomerId)
        });

foreach (var result in query)
    Console.WriteLine("{0} -> {1}", result.Name, result.NumOrders);

Answer 2

您基本上需要在Customer表和Customer Order表之间进行LEFT JOIN，然后根据该结果进行分组，以计算每个客户的订单。

假设你有这样的课程

public class CustomerOrder
{
  public int CustomerId { set; get; }
  public int? OrderId { set; get; }
}

此类用于存储左连接结果的每个项目

现在，您需要首先执行LEFT JOIN并将其结果投影到CustomerOrder类对象列表中，然后在其上执行GroupBy

var usersWithCount = (from c in db.Customers
    join o in db.Orders on c.CustomerId equals o.CustomerId 
    into result
    from sub in result.DefaultIfEmpty()
         select new CustomerOrder{  OrderId= sub!=null ? sub.OrderId :(int?) null,
                                    CustomerId = u.CustomerId }
) // Here we have the left join result.
.GroupBy(g => g.CustomerId , (k, orders) => new { 
                    CustomerId = k, 
                    OrderCount = orders.Count(t=>t.OrderId !=null)}).ToList();

usersWithCount中存储的结果将是具有CustomerId和OrderCount属性的匿名对象的集合。

Answer 3

var Query1 = from c in Customers
     join o in Orders on c.CustomerId equals o.CustomerId into OrdersGroup
    from item in OrdersGroup.DefaultIfEmpty(new Order { OrderId = 0, CustomerId = 0 })                      
    select new {CustomerID = c.CustomerId, OrderID = (item == null ? 0 : item.OrderId)};

这将返回所有客户（即使他们没有订单）和订单

https://dotnetfiddle.net/BoHx2d

LINQ查询 - 显示客户的订单数量，包括没有订单的客户

3 个答案: