Question

entity framework怎么进行一对多的联合查询

Answer 1

以下示例针对 SalesOrderHeader 表和 SalesOrderDetail 表执行 GroupJoin 以查找每个客户的订单数。组联接等效于左外部联接，它返回第一个（左侧）数据源的每个元素（即使其他数据源中没有关联元素）。
using (AdventureWorksEntities AWEntities = new AdventureWorksEntities())
{
ObjectQuery<SalesOrderHeader> orders = AWEntities.SalesOrderHeader;
ObjectQuery<SalesOrderDetail> details = AWEntities.SalesOrderDetail;

var query =
from order in orders
join detail in details
on order.SalesOrderID
equals detail.SalesOrderID into orderGroup
select new
{
CustomerID = order.SalesOrderID,
OrderCount = orderGroup.Count()
};

foreach (var order in query)
{
Console.WriteLine("CustomerID: {0} Orders Count: {1}",
order.CustomerID,
order.OrderCount);
}
}