Join操作

適用場景：在我們表關系中有一對一關系，一對多關系，多對多關系等。對各個表之間的關系，就用這些實現對多個表的操作。

說明：在Join操作中，分別為Join(Join查詢), SelectMany(Select一對多選擇)和GroupJoin(分組Join查詢)。
該擴展方法對兩個序列中鍵匹配的元素進行inner join操作

SelectMany

說明：我們在寫查詢語句時，如果被翻譯成SelectMany需要滿足2個條件。1：查詢語句中沒有join和into，2：必須出現EntitySet。在我們表關系中有一對一關系，一對多關系，多對多關系等，下面分別介紹一下。

1.1 to Many關系：

var q =??? from c in db.Customers??? from o in c.Orders??? where c.City == "London"??? select o;

語句描述：Customers與Orders是一對多關系。即Orders在Customers類中以EntitySet形式出現。所以第二個from是從c.Orders而不是db.Orders里進行篩選。

var q =??? from p in db.Products??? where p.Supplier.Country == "USA" && p.UnitsInStock == 0??? select p;

語句描述：這一句使用了p.Supplier.Country條件，間接關聯了Supplier表。生成SQL語句為：

SELECT [t0].[ProductID], [t0].[ProductName], [t0].[SupplierID],[t0].[CategoryID], [t0].[QuantityPerUnit], [t0].[UnitPrice], [t0].[UnitsInStock], [t0].[UnitsOnOrder],[t0].[ReorderLevel], [t0].[Discontinued]FROM [dbo].[Products] AS [t0]LEFT OUTER JOIN [dbo].[Suppliers] AS [t1] ON [t1].[SupplierID] = [t0].[SupplierID]WHERE ([t1].[Country] = @p0) AND ([t0].[UnitsInStock] = @p1)-- @p0: Input NVarChar (Size = 3; Prec = 0; Scale = 0) [USA]-- @p1: Input Int (Size = 0; Prec = 0; Scale = 0) [0]

2.Many to Many關系：

var q =??? from e in db.Employees??? from et in e.EmployeeTerritories??? where e.City == "Seattle"??? select new {e.FirstName, e.LastName, et.Territory.TerritoryDescription};

說明：多對多關系一般會涉及三個表(如果有一個表是自關聯的，那有可能只有2個表)。這一句語句涉及Employees, EmployeeTerritories, Territories三個表。它們的關系是1：M：1。Employees和Territories沒有很明確的關系。

語句描述：這條生成SQL語句為：
SELECT [t0].[FirstName], [t0].[LastName], [t2].[TerritoryDescription]
FROM [dbo].[Employees] AS [t0]
CROSS JOIN [dbo].[EmployeeTerritories] AS [t1]
INNER JOIN [dbo].[Territories] AS [t2] ON [t2].[TerritoryID] = [t1].[TerritoryID]
WHERE ([t0].[City] = @p0) AND ([t1].[EmployeeID] = [t0].[EmployeeID])
-- @p0: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Seattle]

3.自關聯關系：

var q =??? from e1 in db.Employees??? from e2 in e1.Employees??? where e1.City == e2.City??? select new {??????? FirstName1 = e1.FirstName, LastName1 = e1.LastName,??????? FirstName2 = e2.FirstName, LastName2 = e2.LastName,??????? e1.City??? };

生成SQL語句為：
SELECT [t0].[FirstName] AS [FirstName1], [t0].[LastName] AS [LastName1], [t1].[FirstName] AS [FirstName2], [t1].[LastName] AS [LastName2], [t0].[City]
FROM [dbo].[Employees] AS [t0], [dbo].[Employees] AS [t1]
WHERE ([t0].[City] = [t1].[City]) AND ([t1].[ReportsTo] = [t0].[EmployeeID])

GroupJoin

像上面所說的，沒有join和into，被翻譯成SelectMany，同時有join和into時，那么就被翻譯為GroupJoin。在這里into的概念是對其結果進行重新命名。

1.Two way join(兩個表聯合查詢)

var q =??? from c in db.Customers??? join o in db.Orders on c.CustomerID equals o.CustomerID into orders??? select new {c.ContactName, OrderCount = orders.Count()};

說明：在一對多關系中，左邊是1，它每條記錄為c(from c in db.Customers)，右邊是Many，其每條記錄叫做o ( join o in db.Orders )，每對應左邊的一個c，就會有一組o，那這一組o，就叫做orders，也就是說，我們把一組o命名為orders，這就是into用途。這也就是為什么在select語句中，orders可以調用聚合函數Count。在T-SQL中，使用其內嵌的T-SQL返回值作為字段值。如圖所示：

生成SQL語句為：
SELECT [t0].[ContactName], (
??? SELECT COUNT(*)
??? FROM [dbo].[Orders] AS [t1]
??? WHERE [t0].[CustomerID] = [t1].[CustomerID]
) AS [OrderCount]
FROM [dbo].[Customers] AS [t0]

2.There way join(三個表聯合查詢)

var q =??? from c in db.Customers??? join o in db.Orders on c.CustomerID equals o.CustomerID into ords??? join e in db.Employees on c.City equals e.City into emps??? select new {c.ContactName, ords=ords.Count(), emps=emps.Count()};

生成SQL語句為：
SELECT [t0].[ContactName], (
SELECT COUNT(*)
FROM [dbo].[Orders] AS [t1]
WHERE [t0].[CustomerID] = [t1].[CustomerID]
) AS [ords], (
SELECT COUNT(*)
FROM [dbo].[Employees] AS [t2]
WHERE [t0].[City] = [t2].[City]
) AS [emps]
FROM [dbo].[Customers] AS [t0]

3.Left Outer Join

var q =??? from e in db.Employees??? join o in db.Orders on e equals o.Employee into ords??? from o in ords.DefaultIfEmpty()??? select new {e.FirstName, e.LastName, Order = o};

說明：以Employees左表，Orders右表，Orders 表中為空時，用null值填充。Join的結果重命名ords，使用DefaultIfEmpty()函數對其再次查詢。其最后的結果中有個Order，因為from o in ords.DefaultIfEmpty() 是對ords組再一次遍歷，所以，最后結果中的Order并不是一個集合。但是，如果沒有from o in ords.DefaultIfEmpty() 這句，最后的select語句寫成select new { e.FirstName, e.LastName, Order = ords }的話，那么Order就是一個集合。

4.Projected let assignment

說明：let語句是重命名。let位于第一個from和select語句之間。

var q =??? from c in db.Customers??? join o in db.Orders on c.CustomerID equals o.CustomerID into ords??? let z = c.City + c.Country??? from o in ords??? select new {c.ContactName, o.OrderID, z};

5.Composite Key(組合鍵)

var q =??? from o in db.Orders??? from p in db.Products??? join d in db.OrderDetails??????? on new {o.OrderID, p.ProductID} equals new {d.OrderID, d.ProductID}??????? into details??? from d in details??? select new {o.OrderID, p.ProductID, d.UnitPrice};

說明：使用三個表，并且用匿名類來表示它們之間的關系。它們之間的關系不能用一個鍵描述清楚，所以用匿名類，來表示組合鍵。還有一種是兩個表之間是用組合鍵表示關系的，不需要使用匿名類。

6.Nullable/Nonnullable Key Relationship

var q =??? from o in db.Orders??? join e in db.Employees??????? on o.EmployeeID equals (int?)e.EmployeeID into emps??? from e in emps??? select new {o.OrderID, e.FirstName};

Order By操作

適用場景：對查詢出的語句進行排序，比如按時間排序等等。

說明：按指定表達式對集合排序；延遲，默認是升序，加上descending表示降序，對應的擴展方法是OrderBy和OrderByDescending

1.簡單形式

說明：默認為升序

var q =??? from e in db.Employees??? orderby e.HireDate??? select e;

2.帶條件形式

注意：Where和Order By的順序并不重要。而在T-SQL中，Where和Order By有嚴格的位置限制。

var q =??? from o in db.Orders??? where o.ShipCity == "London"??? orderby o.Freight??? select o;

3.降序排序

var q =??? from p in db.Products??? orderby p.UnitPrice descending??? select p;

4.ThenBy

說明：按多個表達式進行排序，例如先按City排序，當City相同時，按ContactName排序。

var q =??? from c in db.Customers??? orderby c.City, c.ContactName??? select c;

說明：這一句用Lambda表達式像這樣寫：

var q = db.Customers.OrderBy(c => c.City).ThenBy(c => c.ContactName).ToList();

在T-SQL中沒有ThenBy語句，其依然翻譯為OrderBy，所以也可以用下面語句來表達：

var q = db.Customers.OrderBy(c => c.ContactName).OrderBy(c => c.City).ToList();

所要注意的是，多個OrderBy操作時，級連方式是按逆序。對于降序的，用相應的降序操作符替換即可。

var q = db.Customers.OrderByDescending(c => c.City).ThenByDescending(c => c.ContactName).ToList();

需要說明的是，OrderBy操作，不支持按type排序，也不支持匿名類。比如

var q = db.Customers.OrderBy(c => new {c.City,c.ContactName}).ToList();

會被拋出異常。錯誤是前面的操作有匿名類，再跟OrderBy時，比較的是類別。比如

var q = db.Customers.Select(c => new { c.City, c.Address }).OrderBy(c => c).ToList();

如果你想使用OrderBy(c => c)，其前提條件是，前面步驟中，所產生的對象的類別必須為C#語言的基本類型。比如下句，這里City為string類型。

var q = db.Customers.Select(c=>c.City).OrderBy(c => c).ToList();

5.ThenByDescending

這兩個擴展方式都是用在OrderBy/OrderByDescending之后的，第一個ThenBy/ThenByDescending擴展方法作為第二位排序依據，第二個ThenBy/ThenByDescending則作為第三位排序依據，以此類推

var q =??? from o in db.Orders??? where o.EmployeeID == 1??? orderby o.ShipCountry, o.Freight descending??? select o;

6.帶GroupBy形式

var categories =??? from p in db.Products??? group p by p.CategoryID into g??? orderby g.Key??? select new {??????? g.Key,??????? MostExpensiveProducts =??????????? from p2 in g??????????? where p2.UnitPrice == g.Max(p3 => p3.UnitPrice)??????????? select p2??? }; 與50位技術專家面對面20年技術見證，附贈技術全景圖

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