EntityFramework查詢--聯合查詢(Join,GroupJoin)

首先我們先看一下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);

第一個參數outer就是你連接的左面的集合，第二個inner是你要與之連接的集合，第三個outerKeySelector就是要用outer的哪個鍵來進行連接，第四個innerKeySelector同理，最后一個就是返回的類型。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 });

上面的代碼對應的SQL語句如下：

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]

很明顯是用的內連接方式，但是會不會有其他方式呢？那么再來一段代碼看看：

var data = dbContext.Employees.Join(dbContext.Orders, e => e.EmployeeID, o => o.EmployeeID, (e, o) => new { EmployeeID = e.EmployeeID, OrderID = o.OrderID });

對應的SQL語句如下：

SELECT 
    [Extent1].[EmployeeID] AS [EmployeeID], 
    [Extent1].[OrderID] AS [OrderID]
FROM [dbo].[Orders] AS [Extent1]
WHERE [Extent1].[EmployeeID] IS NOT NULL

看到了吧，沒有inner join了，本意是要查找Employee對應的訂單，但是上面的代碼只返回了EmpoloyeeID和OrderID，因為查找Employee對應的訂單，本來就可以直接在Orders中查找的，所以EF還是對此代碼做了優化，免去了inner join。

既然連接已經寫好了，那么要對返回的結果進行篩選你可以調用Where等擴展方法了。

接下來看看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);

來看個例子：Products表有一個CategoryID的屬性，但是Category中並沒要存儲對應的ProductID，如果要查找Category下的Product我們該怎么做呢？如下：

var data = dbContext.Categories.GroupJoin(dbContext.Products, c => c.CategoryID, p => p.CategoryID, (c, p) => new { CategoryID = c.CategoryID, ProductList = p });

對應的SQL語句如下：

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