首先我们先看一下Join
public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector);
第一个参数outer就是你连接的左面的集合，第二个inner是你要与之连接的集合，第三个outerKeySelector就是要用outer的哪个键来进行连接，第四个innerKeySelector同理，最后一个就是返回的类型。用法如下：
NorthwindEntities dbContext = new NorthwindEntities();
var data = dbContext.Employees.Join(dbContext.Orders, e => e.EmployeeID, o => o.EmployeeID, (e, o) => new { EmployeeID = e.EmployeeID, OrderID = o.OrderID, Address = o.ShipAddress, EmployeeAddress = e.Address });
上面的代码对应的SQL语句如下：
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]
很明显是用的内连接方式，但是会不会有其他方式呢？那么再来一段代码看看：
var data = dbContext.Employees.Join(dbContext.Orders, e => e.EmployeeID, o => o.EmployeeID, (e, o) => new { EmployeeID = e.EmployeeID, OrderID = o.OrderID });
对应的SQL语句如下：
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[OrderID] AS [OrderID]
FROM [dbo].[Orders] AS [Extent1]
WHERE [Extent1].[EmployeeID] IS NOT NULL
看到了吧，没有inner join了，本意是要查找Employee对应的订单，但是上面的代码只返回了EmpoloyeeID和OrderID，因为查找Employee对应的订单，本来就可以直接在Orders中查找的，所以EF还是对此代码做了优化，免去了inner join。
既然连接已经写好了，那么要对返回的结果进行筛选你可以调用Where等扩展方法了。
接下来看看GroupJoin
public static IQueryable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IQueryable<TOuter> outer, IEnumerable<TInner> inner, Expression<Func<TOuter, TKey>> outerKeySelector, Expression<Func<TInner, TKey>> innerKeySelector, Expression<Func<TOuter, IEnumerable<TInner>, TResult>> resultSelector);
来看个例子：Products表有一个CategoryID的属性，但是Category中并没要存储对应的ProductID，如果要查找Category下的Product我们该怎么做呢？如下：
var data = dbContext.Categories.GroupJoin(dbContext.Products, c => c.CategoryID, p => p.CategoryID, (c, p) => new { CategoryID = c.CategoryID, ProductList = p });
对应的SQL语句如下：
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 AS int) 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