Question

这是一个简单的LINQ查询（基于NorthWind），它返回一个Customers列表。每个客户都包含订单列表。

from c in Customers
join o in Orders on c.CustomerID equals o.CustomerID into CO
select new {c, CO}

这很好用，生成的SQL也没问题。现在我想更进一步。我希望每个Order对象都包含OrderDetails列表。我使用以下查询：

from c in Customers
join od in (
    from o in Orders 
    join od in OrderDetails on o.OrderID equals od.OrderID into OD 
    select new { o.CustomerID, o, OD }
)
on c.CustomerID equals od.CustomerID into COD
select new { c, COD }

此查询有效，但会生成可怕的SQL。为每个客户发出单独的查询。当你查看我们的lambda代码时：

Customers
   .GroupJoin (
      Orders
         .GroupJoin (
            OrderDetails, 
            o => o.OrderID, 
            od => od.OrderID, 
            (o, OD) => 
               new  
               {
                  CustomerID = o.CustomerID, 
                  o = o, 
                  OD = OD
               }
         ), 
      c => c.CustomerID, 
      od => od.CustomerID, 
      (c, COD) => 
         new  
         {
            c = c, 
            COD = COD
         }
   )

嵌套的GroupJoins似乎是多个SQL stataments的原因。但是，我尝试了各种组合而没有成功。有什么想法吗？

编辑：我可能有点不清楚我想要实现的目标。我希望OrderDetail对象是Order对象的属性，而Order对象又是Customer对象的属性。我不想订购＆amp; OrderDetail是Customer的属性。我想获得独特客户的名单。对于每个客户，我期望订单列表和每个订单我想要一个OrderDetails列表。我希望层次结构比原始查询更深一层。

Answer 1

如果要强制执行单个查询，则可以在客户端执行分组，而不是在服务器端执行分组：

from a in (from c in Customers
    join o in Orders on c.CustomerID equals o.CustomerID
    join od in OrderDetails on o.OrderID equals od.OrderID
    select new {c, o, od}).AsEnumerable()
group a by a.c into g
select new { Customer = g.Key, Orders = g.Select(o => o.o) , OrderDetails = g.Select(od => od.od)}

生成的SQL是：

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax], [t1].[OrderID], [t1].[CustomerID] AS [CustomerID2], [t1].[EmployeeID], [t1].[OrderDate], [t1].[RequiredDate], [t1].[ShippedDate], [t1].[ShipVia], [t1].[Freight], [t1].[ShipName], [t1].[ShipAddress], [t1].[ShipCity], [t1].[ShipRegion], [t1].[ShipPostalCode], [t1].[ShipCountry], [t2].[OrderID] AS [OrderID2], [t2].[ProductID], [t2].[UnitPrice], [t2].[Quantity], [t2].[Discount]
FROM [Customers] AS [t0]
INNER JOIN [Orders] AS [t1] ON [t0].[CustomerID] = [t1].[CustomerID]
INNER JOIN [Order Details] AS [t2] ON [t1].[OrderID] = [t2].[OrderID]

请记住，这比多次查询不更快，同时会增加网络和服务器负载。

我的建议是使用如下查询：

DataLoadOptions opt = new DataLoadOptions();
opt.LoadWith<Orders>(o => o.OrderDetails);
this.LoadOptions = opt;

from c in Customers
select new {c, Orders = c.Orders, OrderDetails = c.Orders.SelectMany( o=> o.OrderDetails)}

Answer 2

为什么不使用多个连接：

from c in Customers
join o in Orders on c.CustomerID equals o.CustomerID
join od in OrderDetails on o.OrderID equals od.OrderID
select new {c, o, od}

Answer 3

您可能尝试加入服务器上的OrderDetails，然后在客户端上“重新组合”以获得完整的分层数据结构：

var q = from c in Customers
        join o in Orders on c.CustomerID equals o.CustomerID 
        join od in OrderDetails on o.OrderID equals od.OrderID into OD 
        select new { c, o, OD };

var res = from x in q.AsEnumerable()
          group x by x.c.CustomerID into g
          select new
          {
              Customer = g.First().c,
              Orders = g.Select(y => new
                       {
                           Order = y.o,
                           OrderDetails = y.OD
                       })
          };

Answer 4

看来linq2sql无法有效地处理多个表跨度（按设计？）：

http://codebetter.com/blogs/david.hayden/archive/2007/08/06/linq-to-sql-query-tuning-appears-to-break-down-in-more-advanced-scenarios.aspx

http://social.msdn.microsoft.com/forums/en-US/adodotnetentityframework/thread/23bc2a58-ff1e-4857-81ae-507bb1a2754d/

LINQ中的分层查询

4 个答案: