Linq Sub-Select
如何在LINQ中编写子选择。
如果我有一个客户列表和一个订单列表,我希望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
怎么样:
var res = from c in customers where !orders.Select(o => o.CustomerID).Contains(c.CustomerID) select c;
另一种选择是使用:
var res = from c in customers join o in orders on c.CustomerID equals o.customerID into customerOrders where customerOrders.Count() == 0 select c;
您使用LINQ to SQL还是别的什么,顺便问一下? 不同的口味可能有不同的“最佳”方式
如果这是由数据库支持的,请尝试使用导航属性(如果已定义它们):
var res = from c in customers where !c.Orders.Any() select c;
在Northwind上,这会生成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] ))
这项工作做得很好。
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();
var res = (from c in orders where c.CustomerID == null select c.Customers).ToList();
或者除了()