LINQ标准查询操作符（一）——select、SelectMany、Where、OrderBy、OrderByDescending、ThenBy、ThenByDescending和Reverse

一、投影操作符

1. Select

Select操作符对单个序列或集合中的值进行投影。下面的示例中使用select从序列中返回Employee表的所有列：

    //查询语法
    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].[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%' 

    //方法语法
    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%'

可以简写为：
var qq =
        db.Employees
        .Where(e => e.FirstName.StartsWith("M"))
        .ToList();

当然，你也可以返回单个列，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%'

如果像上面这样，只查询单列，则返回类似于无名称的数组，并不能用于生成Json。

如果想返回单列匿名类，应

var query1 =
        db.Employees.Where(e => e.FirstName.StartsWith("M"))
        .Select(e => new { e.FirstName })
        .ToList();

返回结果为：

你也可以返回序列中的某几列，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%'

查询多列，返回的是一个匿名类

2. SelectMany

SelectMany操作符提供了将多个from子句组合起来的功能，它将每个对象的结果合并成单个序列。下面是一个示例：

var query =
    (from e in db.Employees
     from o in e.Orders
     select o).ToList();
//方法语法
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

场景：Employee与Order为0或1对多的关系，也就是Order的EmployeeID可以为null

从生成的sql语句也可以看到，只查询出了Order的EmployeeID不为null的所有Order记录。

二、限制操作符

Where是限制操作符，它将过滤标准应用在序列上，按照提供的逻辑对序列中的数据进行过滤。

Where操作符不启动查询的执行。当开始对序列进行遍历时查询才开始执行，此时过滤条件将被应用到查询中。Where操作符的使用方法已经在第一节中出现过，这里不再冗述。

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操作符将序列中的元素按照升序排列。下面的示例演示了这一点：

//查询语法
var query =
    (from e in db.Employees
     orderby e.FirstName
     select e).ToList();
//方法语法
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

因为未使用Select，所以返回的为动态包装类。

这里可以使用OrderBy的重载方法OrderBy(Func<T,TKey>,IComparer<Tkey>)来指定序列的排序方式。

2. OrderByDescending

OrderByDescending操作符将序列中的元素按照降序排列。用法与OrderBy相同，这里不再演示。

3. ThenBy

ThenBy操作符实现按照次关键字对序列进行升序排列。此操作符的查询语法与方法语法略有不同，以下代码演示了这一点：

//查询语法
var query =
    (from e in db.Employees
     orderby e.FirstName, e.LastName
     select e).ToList();
//方法语法
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操作符实现按照次关键字对序列进行降序排列。此操作符的查询语法与方法语法略有不同，以下代码演示了这一点：

//查询语法
var query =
    from e in db.Employees
    orderby e.FirstName,e.LastName descending
    select e;
//方法语法
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将会把序列中的元素按照从后到前的循序反转。需要注意的是，Reverse方法的返回值是void，以下代码演示了这一点

//方法语法
var q =
    db.Employees
    .Select(e => e.FirstName)
    .ToList();
q.Reverse();