LINQ標準查詢操作符（一）——select、SelectMany、Where、OrderBy、OrderByDescending、ThenBy、ThenByDescending和Reverse
阿新 • • 發佈：2019-04-12
extent art 相同 開始 ima 方法 降序排序 ext extension 
一、投影操作符
1. Select
Select操作符對單個序列或集合中的值進行投影。下面的示例中使用select從序列中返回Employee表的所有列：
//查詢語法
var query =
from e in db.Employees
where e.FirstName.StartsWith("M")
select e;
生成的sql:
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[LastName] AS [LastName],
[Extent1].[Title] AS [Title],
[Extent1].[TitleOfCourtesy] AS [TitleOfCourtesy],
[Extent1].[BirthDate] AS [BirthDate],
[Extent1].[HireDate] AS [HireDate],
[Extent1].[Address] AS [Address],
[Extent1].[City] AS [City],
[Extent1].[Region] AS [Region],
[Extent1].[Country] AS [Country],
[Extent1].[HomePhone] AS [HomePhone],
[Extent1].[Extension] AS [Extension],
[Extent1].[Photo] AS [Photo],
[Extent1].[Notes] AS [Notes],
[Extent1].[ReportsTo] AS [ReportsTo],
[Extent1].[PhotoPath] AS [PhotoPath]
WHERE [Extent1].[FirstName] LIKE N‘M%‘
//方法語法
var q =
db.Employees
.Where(e => e.FirstName.StartsWith("M"))
.Select(e => e);
生成的sql:
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[LastName] AS [LastName],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[Title] AS [Title],
[Extent1].[TitleOfCourtesy] AS [TitleOfCourtesy],
[Extent1].[BirthDate] AS [BirthDate],
[Extent1].[HireDate] AS [HireDate],
[Extent1].[Address] AS [Address],
[Extent1].[City] AS [City],
[Extent1].[Region] AS [Region],
[Extent1].[PostalCode] AS [PostalCode],
[Extent1].[Country] AS [Country],
[Extent1].[HomePhone] AS [HomePhone],
[Extent1].[Extension] AS [Extension],
[Extent1].[Photo] AS [Photo],
[Extent1].[Notes] AS [Notes],
[Extent1].[ReportsTo] AS [ReportsTo],
[Extent1].[PhotoPath] AS [PhotoPath]
FROM [dbo].[Employees] AS [Extent1]
WHERE [Extent1].[FirstName] LIKE N‘M%‘
可以簡寫為：
var qq =
db.Employees
.Where(e => e.FirstName.StartsWith("M"))
.ToList();
=========================================================
當然，你也可以返回單個列，例如：
var query =
from e in db.Employees
where e.FirstName.StartsWith("M")
select e.FirstName;
var query1 =
db.Employees.Where(e => e.FirstName.StartsWith("M"))
.Select(e => e.FirstName);
生成的sql:
SELECT
[Extent1].[FirstName] AS [FirstName]
FROM [dbo].[Employees] AS [Extent1]
WHERE [Extent1].[FirstName] LIKE N‘M%‘
如果像上面這樣，只查詢單列，則返回類似於無名稱的數組，並不能用於生成Json。
如果想返回單列匿名類，應
var query1 =
db.Employees.Where(e => e.FirstName.StartsWith("M"))
.Select(e => new { e.FirstName })
.ToList();
返回結果為：
==========================================================
你也可以返回序列中的某幾列，例如：
var query =
(from e in db.Employees
where e.FirstName.StartsWith("M")
select new
{
e.FirstName,
e.LastName,
e.Title
}).ToList();
var query1 =
db.Employees.Where(e => e.FirstName.StartsWith("M"))
.Select(e => new
{
e.FirstName,
e.LastName,
e.Title
})
.ToList();
生成的sql:
SELECT
1 AS [C1],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[LastName] AS [LastName],
[Extent1].[Title] AS [Title]
FROM [dbo].[Employees] AS [Extent1]
WHERE [Extent1].[FirstName] LIKE N‘M%‘
查詢多列，返回的是一個匿名類
2. SelectMany
SelectMany操作符提供了將多個from子句組合起來的功能，它將每個對象的結果合並成單個序列。下面是一個示例：
var query =
(from e in db.Employees
from o in e.Orders
select o).ToList();
//方法語法
var q =
db.Employees
.SelectMany(e => e.Orders)
.ToList();
生成的sql:
SELECT
[Extent1].[OrderID] AS [OrderID],
[Extent1].[CustomerID] AS [CustomerID],
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[OrderDate] AS [OrderDate],
[Extent1].[RequiredDate] AS [RequiredDate],
[Extent1].[ShippedDate] AS [ShippedDate],
[Extent1].[ShipVia] AS [ShipVia],
[Extent1].[Freight] AS [Freight],
[Extent1].[ShipName] AS [ShipName],
[Extent1].[ShipAddress] AS [ShipAddress],
[Extent1].[ShipCity] AS [ShipCity],
[Extent1].[ShipRegion] AS [ShipRegion],
[Extent1].[ShipPostalCode] AS [ShipPostalCode],
[Extent1].[ShipCountry] AS [ShipCountry]
FROM [dbo].[Orders] AS [Extent1]
WHERE [Extent1].[EmployeeID] IS NOT NULL
場景：Employee與Order為0或1對多的關系，也就是Order的EmployeeID可以為null
從生成的sql語句也可以看到，只查詢出了Order的EmployeeID不為null的所有Order記錄。
二、限制操作符
Where是限制操作符，它將過濾標準應用在序列上，按照提供的邏輯對序列中的數據進行過濾。
Where操作符不啟動查詢的執行。當開始對序列進行遍歷時查詢才開始執行，此時過濾條件將被應用到查詢中。Where操作符的使用方法已經在第一節中出現過，這裏不再冗述。
三、排序操作符
排序操作符，包括OrderBy、OrderByDescending、ThenBy、ThenByDescending和Reverse，提供了升序或者降序排序。
1. OrderBy
OrderBy操作符將序列中的元素按照升序排列。下面的示例演示了這一點：
//查詢語法
var query =
(from e in db.Employees
orderby e.FirstName
select e).ToList();
//方法語法
var q =
db.Employees
.OrderBy(e => e.FirstName)
.ToList();
生成的sql:
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[LastName] AS [LastName],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[Title] AS [Title],
[Extent1].[TitleOfCourtesy] AS [TitleOfCourtesy],
[Extent1].[BirthDate] AS [BirthDate],
[Extent1].[HireDate] AS [HireDate],
[Extent1].[Address] AS [Address],
[Extent1].[City] AS [City],
[Extent1].[Region] AS [Region],
[Extent1].[PostalCode] AS [PostalCode],
[Extent1].[Country] AS [Country],
[Extent1].[HomePhone] AS [HomePhone],
[Extent1].[Extension] AS [Extension],
[Extent1].[Photo] AS [Photo],
[Extent1].[Notes] AS [Notes],
[Extent1].[ReportsTo] AS [ReportsTo],
[Extent1].[PhotoPath] AS [PhotoPath]
FROM [dbo].[Employees] AS [Extent1]
ORDER BY [Extent1].[FirstName] ASC
因為未使用Select，所以返回的為動態包裝類。
這裏可以使用OrderBy的重載方法OrderBy(Func<T,TKey>,IComparer<Tkey>)來指定序列的排序方式。
2. OrderByDescending
OrderByDescending操作符將序列中的元素按照降序排列。用法與OrderBy相同，這裏不再演示。
3. ThenBy
ThenBy操作符實現按照次關鍵字對序列進行升序排列。此操作符的查詢語法與方法語法略有不同，以下代碼演示了這一點：
//查詢語法
var query =
(from e in db.Employees
orderby e.FirstName, e.LastName
select e).ToList();
//方法語法
var q =
db.Employees
.OrderBy(e => e.FirstName)
.ThenBy(e => e.LastName)
.ToList();
生成的sql:
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[LastName] AS [LastName],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[Title] AS [Title],
[Extent1].[TitleOfCourtesy] AS [TitleOfCourtesy],
[Extent1].[BirthDate] AS [BirthDate],
[Extent1].[HireDate] AS [HireDate],
[Extent1].[Address] AS [Address],
[Extent1].[City] AS [City],
[Extent1].[Region] AS [Region],
[Extent1].[PostalCode] AS [PostalCode],
[Extent1].[Country] AS [Country],
[Extent1].[HomePhone] AS [HomePhone],
[Extent1].[Extension] AS [Extension],
[Extent1].[Photo] AS [Photo],
[Extent1].[Notes] AS [Notes],
[Extent1].[ReportsTo] AS [ReportsTo],
[Extent1].[PhotoPath] AS [PhotoPath]
FROM [dbo].[Employees] AS [Extent1]
ORDER BY [Extent1].[FirstName] ASC, [Extent1].[LastName] ASC
4. ThenByDescending
ThenByDescending操作符實現按照次關鍵字對序列進行降序排列。此操作符的查詢語法與方法語法略有不同，以下代碼演示了這一點：
//查詢語法
var query =
from e in db.Employees
orderby e.FirstName,e.LastName descending
select e;
//方法語法
var q =
db.Employees
.OrderBy(e => e.FirstName)
.ThenByDescending(e => e.LastName)
.Select(e => e);
foreach (var item in query)
{
Console.WriteLine(item.FirstName);
}
生成的sql:
SELECT
[Extent1].[EmployeeID] AS [EmployeeID],
[Extent1].[LastName] AS [LastName],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[Title] AS [Title],
[Extent1].[TitleOfCourtesy] AS [TitleOfCourtesy],
[Extent1].[BirthDate] AS [BirthDate],
[Extent1].[HireDate] AS [HireDate],
[Extent1].[Address] AS [Address],
[Extent1].[City] AS [City],
[Extent1].[Region] AS [Region],
[Extent1].[PostalCode] AS [PostalCode],
[Extent1].[Country] AS [Country],
[Extent1].[HomePhone] AS [HomePhone],
[Extent1].[Extension] AS [Extension],
[Extent1].[Photo] AS [Photo],
[Extent1].[Notes] AS [Notes],
[Extent1].[ReportsTo] AS [ReportsTo],
[Extent1].[PhotoPath] AS [PhotoPath]
FROM [dbo].[Employees] AS [Extent1]
ORDER BY [Extent1].[FirstName] ASC, [Extent1].[LastName] DESC
5. Reverse
Reverse將會把序列中的元素按照從後到前的循序反轉。需要註意的是，Reverse方法的返回值是void，以下代碼演示了這一點
//方法語法
var q =
db.Employees
.Select(e => e.FirstName)
.ToList();
q.Reverse();
LINQ標準查詢操作符（一）——select、SelectMany、Where、OrderBy、OrderByDescending、ThenBy、ThenByDescending和Reverse