Linq子查询

Question

如何在LINQ中编写子查询。

如果我有一个客户列表和一个订单列表，我想要所有没有订单的客户。

这是我的伪代码尝试：

    var  res = from c in customers 
where c.CustomerID ! in (from o in orders select o.CustomerID) 
select c

- Dan

Answer 1

怎么样：

var res = from c in customers 
          where !orders.Select(o => o.CustomerID).Contains(c.CustomerID)
          select c;

另一个选择是使用：

var res = from c in customers
          join o in orders 
               on c.CustomerID equals o.customerID 
               into customerOrders
          where customerOrders.Count() == 0
          select c;

你正在使用LINQ to SQL还是其他东西？不同的工具可能有不同的最佳实践。

- Jon Skeet

---

在可读性方面，使用Any()代替Count()是否略优？我正在阅读Bill Wagner的《More Effective C#》时遇到了这个建议。 - Ray Booysen

是的，很有可能。有很多方法可以做到这一点。可以说，也许有一个Empty()或None()扩展方法会很好，它是Any()方法的相反。 - Jon Skeet

Answer 2

如果这是基于数据库的，试着使用导航属性（如果你已经定义了它们）：

var res = from c in customers
          where !c.Orders.Any()
          select c;

在Northwind上，这将生成TSQL：

SELECT /* columns snipped */
FROM [dbo].[Customers] AS [t0]
WHERE NOT (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [dbo].[Orders] AS [t1]
    WHERE [t1].[CustomerID] = [t0].[CustomerID]
    ))

它能够很好地完成工作。

- Marc Gravell

Answer 3

            var result = (from planinfo in db.mst_pointplan_info
                                                           join entityType in db.mst_entity_type
                                                           on planinfo.entityId equals entityType.id
                                                           where planinfo.entityId == entityId
                                                           && planinfo.is_deleted != true
                                                           && planinfo.system_id == systemId
                                                           && entityType.enity_enum_id == entityId
                                                           group planinfo by planinfo.package_id into gplan
                                                           select new PackagePointRangeConfigurationResult
                                                           {
                                                               Result = (from planinfo in gplan
                                                                         select new PackagePointRangeResult
                                                                         {
                                                                             PackageId = planinfo.package_id,
                                                                             PointPlanInfo = (from pointPlanInfo in gplan
                                                                                              select new PointPlanInfo
                                                                                              {
                                                                                                  StartRange = planinfo.start_range,
                                                                                                  EndRange = planinfo.end_range,
                                                                                                  IsDiscountAndChargeInPer = planinfo.is_discount_and_charge_in_per,
                                                                                                  Discount = planinfo.discount,
                                                                                                  ServiceCharge = planinfo.servicecharge,
                                                                                                  AtonMerchantShare = planinfo.aton_merchant_share,
                                                                                                  CommunityShare = planinfo.community_share
                                                                                              }).ToList()
                                                                         }).ToList()
                                                           }).FirstOrDefault();

- amit

Answer 4

var  res = (from c in orders where c.CustomerID == null
               select c.Customers).ToList();

或者制造异常()

- omoto