Join操作符
適用場景:在我們表關係中有一對一關係,一對多關係,多對多關係等。對各個表之間的關係,就用這些實現對多個表的操作。
說明:在Join操作中,分別爲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。在我們表關係中有一對一關係,一對多關係,多對多關係等,下面分別介紹一下。
1.一對多關係(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是一對多關係。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裏進行篩選。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語句爲:
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.多對多關係(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:多對多關係一般會涉及三個表(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。它們的關係是1:M:1。Employees和Territories沒有很明確的關係。
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僱員,同時列出其所在地區。這條生成SQL語句爲:
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.自聯接關係:
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僱員,每對中一個僱員隸屬於另一個僱員,且兩個僱員都來自相同城市。生成SQL語句爲:
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,那麼就被翻譯爲GroupJoin。在這裏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並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:在一對多關係中,左邊是1,它每條記錄爲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命名爲orders,這就是into用途。這也就是爲什麼在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返回值作爲字段值。如圖所示:
生成SQL語句爲:
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並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語句爲:
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。在僱員沒有訂單時,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 表中爲空時,用null值填充。Join的結果重命名ords,使用DefaultIfEmpty()函數對其再次查詢。其最後的結果中有個Order,因爲from o in ords.DefaultIfEmpty() 是對ords組再一次遍歷,所以,最後結果中的Order並不是一個集合。但是,如果沒有from o in ords.DefaultIfEmpty() 這句,最後的select語句寫成select new { e.FirstName, e.LastName, Order = ords }的話,那麼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位於第一個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,並且用匿名類來說明:使用三個表,並且用匿名類來表示它們之間的關係。它們之間的關係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係的,不需要使用匿名類。
6.可爲null/不可爲null的鍵關係(Nullable/Nonnullable Key Relationship):
這個實例顯示如何構造一側可爲 null 而另一側不可爲 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
本文來自CSDN博客,轉載請標明出處:http://blog.csdn.net/xiaoyu411502/archive/2009/06/11/4260671.aspx