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。
1.一對多關系(1 to Many):
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。
這個例子在From子句中使用外鍵導航選擇倫敦客戶的所有訂單
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。這個例子在Where子句中使用外鍵導航篩選其供應商在美國且缺貨的產品。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.多對多關系(Many to Many):
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:這個例子在From子句中使用外鍵導航篩選在西雅圖的雇員,同時列出其所在地區。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.自聯接關系:
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 };
語句描述:這個例子在select 子句中使用外鍵導航篩選成對的雇員,每對中一個雇員隸屬于另一個雇員,且兩個雇員都來自相同城市。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.雙向聯接(Two way join):
此示例顯式聯接兩個表并從這兩個表投影出結果:
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]
2.三向聯接(There way join):
此示例顯式聯接三個表并分別從每個表投影出結果:
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.左外部聯接(Left Outer Join):
此示例說明如何通過使用 此示例說明如何通過使用DefaultIfEmpty() 獲取左外部聯接。在雇員沒有訂單時,DefaultIfEmpty()方法返回null:
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.投影的Let賦值(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。
這個例子從聯接投影出最終“Let”表達式:
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.組合鍵(Composite Key):
這個例子顯示帶有組合鍵的聯接:
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.可為null/不可為null的鍵關系(Nullable/Nonnullable Key Relationship):
這個實例顯示如何構造一側可為 null 而另一側不可為 null 的聯接:
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 };
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