用EF很多年了，做联合查询时，只知道linq和lambda两种语法，今天朋友发了一个链接，打开看后发现是EF内置的新的关于联合查询的方法，赶紧抄录下来，以备后用。
现在先把这几种方法，各写一个例子，便于大家参考：
linq:
var re = from o in db.Order
join d in db.OrderDetail
on o.Code equals d.OrderCode
join p in db.Product
on d.ProductID equals p.id
where o.Code == "20150326000096"
select new {
OrderCode = o.Code,
ProductName = p.Name,
SalePrice = p.SalePrice,
ProductQuantity = d.Quantity,
Amount = d.Amount
};
SELECT
[Extent1].[ProductID] AS [ProductID],
[Extent1].[OrderCode] AS [OrderCode],
[Extent2].[Name] AS [Name],
[Extent2].[SalePrice] AS [SalePrice],
[Extent1].[Quantity] AS [Quantity],
[Extent1].[Amount] AS [Amount]
FROM [dbo].[OrderDetails] AS [Extent1]
INNER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[ProductID] = [Extent2].[id]
WHERE ([Extent1].[OrderCode] IS NOT NULL) AND (N'20150326000096' = [Extent1].[OrderCode])
lambda:
re = db.OrderDetail.Where(d => d.OrderCode == "20150326000096").Select(d => new {
OrderCode = d.Order.Code,
ProductName = d.Product.Name,
SalePrice = d.Product.SalePrice,
ProductQuantity = d.Quantity,
Amount = d.Amount
});
SELECT
[Extent1].[ProductID] AS [ProductID],
[Extent1].[OrderCode] AS [OrderCode],
[Extent2].[Name] AS [Name],
[Extent2].[SalePrice] AS [SalePrice],
[Extent1].[Quantity] AS [Quantity],
[Extent1].[Amount] AS [Amount]
FROM [dbo].[OrderDetails] AS [Extent1]
INNER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[ProductID] = [Extent2].[id]
WHERE N'20150326000096' = [Extent1].[OrderCode]
内置join方法：
var re = db.Employees.Join(db.Orders, e => e.EmployeeID, o => o.EmployeeID, (e, o) => new { EmployeeID = e.EmployeeID, OrderID = o.OrderID, Address = o.ShipAddress, EmployeeAddress = e.Address });
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent2].[OrderID] AS [OrderID],
[Extent2].[ShipAddress] AS [ShipAddress],
[Extent1].[Address] AS [Address]
from [dbo].[Employees] AS [Extent1]
INNER JOIN [dbo].[Orders] AS [Extent2] ON [Extent1].[EmployeeID]= [Extent2].[EmployeeID]
内置Join用法2：
var re = db.Employees.Join(db.Orders, e => e.EmployeeID, o => o.EmployeeID, (e, o) => new { EmployeeID = e.EmployeeID, OrderID = o.OrderID });
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[OrderID] AS [OrderID]
FROM [dbo].[Orders] AS [Extent1]
WHERE [Extent1].[EmployeeID] IS NOT NULL
内置的GroupJoin方法：
var re = db.Categories.GroupJoin(db.Products, c => c.CategoryID, p => p.CategoryID, (c, p) => new { CategoryID = c.CategoryID, ProductList = p });
SELECT
[Project1].[CategoryID] AS [CategoryID],
[Project1].[C1] AS [C1],
[Project1].[ProductID] AS [ProductID],
[Project1].[ProductName] AS [ProductName],
[Project1].[SupplierID] AS [SupplierID],
[Project1].[CategoryID1] AS [CategoryID1],
[Project1].[QuantityPerUnit] AS [QuantityPerUnit],
[Project1].[UnitPrice] AS [UnitPrice],
[Project1].[UnitsInStock] AS [UnitsInStock],
[Project1].[UnitsOnOrder] AS [UnitsOnOrder],
[Project1].[ReorderLevel] AS [ReorderLevel],
[Project1].[Discontinued] AS [Discontinued]
from ( SELECT
[Extent1].[CategoryID] AS [CategoryID],
[Extent2].[ProductID] AS [ProductID],
[Extent2].[ProductName] AS [ProductName],
[Extent2].[SupplierID] AS [SupplierID],
[Extent2].[CategoryID] AS [CategoryID1],
[Extent2].[QuantityPerUnit] AS [QuantityPerUnit],
[Extent2].[UnitPrice] AS [UnitPrice],
[Extent2].[UnitsInStock] AS [UnitsInStock],
[Extent2].[UnitsOnOrder] AS [UnitsOnOrder],
[Extent2].[ReorderLevel] AS [ReorderLevel],
[Extent2].[Discontinued] AS [Discontinued],
CASE WHEN ([Extent2].[ProductID] IS NULL) THEN CAST(NULL ASint) ELSE 1 END AS [C1]
FROM [dbo].[Categories] AS [Extent1]
LEFT OUTER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]
) AS [Project1]
ORDER BY [Project1].[CategoryID] ASC, [Project1].[C1] ASC
Join和GroupJoin方法还没用过，代码是照搬的，以后应用中发现了什么再来修改吧，先贴出来备用
参考文章：EntityFramework查询--联合查询(Join,GroupJoin)