sqlce wp from查询语句详解

http://msdn.microsoft.com/zh-cn/library/bb383978(v=vs.110).aspx

from 子句（C# 参考）

Visual Studio 2012

 

 

其他版本

 

 

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查询表达式必须以 from 子句开头。 另外，查询表达式还可以包含子查询，子查询也是以 from 子句开头。 from 子句指定以下内容：

• 将对其运行查询或子查询的数据源。

• 一个本地范围变量，表示源序列中的每个元素。

范围变量和数据源都是强类型。 from 子句中引用的数据源的类型必须为 IEnumerable、IEnumerable<T> 或一种派生类型（如 IQueryable<T>）。

在下面的示例中，numbers 是数据源，而 num 是范围变量。 请注意，这两个变量都是强类型，即使使用了 var 关键字也是如此。

C#

 

class LowNums
{
    static void Main()
    {   
        // A simple data source.
        int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

        // Create the query.
        // lowNums is an IEnumerable<int>
        var lowNums = from num in numbers
            where num < 5
            select num;

        // Execute the query.
        foreach (int i in lowNums)
        {
            Console.Write(i + " ");
        }
    }        
}
// Output: 4 1 3 2 0

范围变量

---

如果数据源实现了 IEnumerable<T>，则编译器可以推断范围变量的类型。 例如，如果数据源的类型为 IEnumerable<Customer>，则推断出范围变量的类型为 Customer。 仅当数据源是非泛型 IEnumerable 类型（如 ArrayList）时，才必须显式指定数据源类型。 有关更多信息，请参见 如何：使用 LINQ 查询 ArrayList。

在上一个示例中，num 被推断为 int 类型。 由于范围变量是强类型，因此可以对其调用方法或者在其他操作中使用它。 例如，可以不编写 select num，而编写 select num.ToString() 使查询表达式返回一个字符串序列而不是整数序列。 或者，也可以编写 select n + 10 使表达式返回序列 14、11、13、12、10。 有关更多信息，请参见 select 子句（C# 参考）。

范围变量类似于 foreach 语句中的迭代变量，只是两者之间有一个非常重要的区别：范围变量从不实际存储来自数据源的数据。 范围变量只是提供了语法上的便利，使查询能够描述执行查询时将发生的事情。 有关更多信息，请参见LINQ 查询简介 (C#)。

复合 from 子句

---

在某些情况下，源序列中的每个元素本身可能是序列，也可能包含序列。 例如，数据源可能是一个 IEnumerable<Student>，其中，序列中的每个 Student 对象都包含一个测验得分列表。 若要访问每个 Student 元素中的内部列表，可以使用复合 from 子句。 该技术类似于使用嵌套的 foreach 语句。 可以向任一 from 子句中添加 where 或 orderby 子句来筛选结果。 下面的示例演示了一个 Student 对象序列，其中每个对象都包含一个表示测验得分的内部整数List。 为了访问该内部列表，此示例使用了复合 from 子句。 如有必要，可在两个 from 子句之间再插入子句。

C#

 

class CompoundFrom
{
    // The element type of the data source.
    public class Student
    {
        public string LastName { get; set; }
        public List<int> Scores {get; set;}
    }

    static void Main()
    {

        // Use a collection initializer to create the data source. Note that 
        // each element in the list contains an inner sequence of scores.
        List<Student> students = new List<Student>
        {
           new Student {LastName="Omelchenko", Scores= new List<int> {97, 72, 81, 60}},
           new Student {LastName="O'Donnell", Scores= new List<int> {75, 84, 91, 39}},
           new Student {LastName="Mortensen", Scores= new List<int> {88, 94, 65, 85}},
           new Student {LastName="Garcia", Scores= new List<int> {97, 89, 85, 82}},
           new Student {LastName="Beebe", Scores= new List<int> {35, 72, 91, 70}} 
        };        

        // Use a compound from to access the inner sequence within each element.
        // Note the similarity to a nested foreach statement.
        var scoreQuery = from student in students
                         from score in student.Scores
                            where score > 90
                            select new { Last = student.LastName, score };

        // Execute the queries.
        Console.WriteLine("scoreQuery:");
        // Rest the mouse pointer on scoreQuery in the following line to 
        // see its type. The type is IEnumerable<'a>, where 'a is an 
        // anonymous type defined as new {string Last, int score}. That is,
        // each instance of this anonymous type has two members, a string 
        // (Last) and an int (score).
        foreach (var student in scoreQuery)
        {
            Console.WriteLine("{0} Score: {1}", student.Last, student.score);
        }

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }       
}
/*
scoreQuery:
Omelchenko Score: 97
O'Donnell Score: 91
Mortensen Score: 94
Garcia Score: 97
Beebe Score: 91
*/

使用多个 from 子句执行联接

---

复合 from 子句用于访问单个数据源中的内部集合。 不过，查询还可以包含多个可从独立数据源生成补充查询的 from 子句。 使用此技术可以执行某些类型的、无法通过使用 join 子句执行的联接操作。

下面的示例演示如何使用两个 from 子句构成两个数据源的完全交叉联接。

C#

 

class CompoundFrom2
{
    static void Main()
    {
        char[] upperCase = { 'A', 'B', 'C' };
        char[] lowerCase = { 'x', 'y', 'z' };

        // The type of joinQuery1 is IEnumerable<'a>, where 'a
        // indicates an anonymous type. This anonymous type has two
        // members, upper and lower, both of type char.
        var joinQuery1 =
            from upper in upperCase
            from lower in lowerCase
            select new { upper, lower };

        // The type of joinQuery2 is IEnumerable<'a>, where 'a
        // indicates an anonymous type. This anonymous type has two
        // members, upper and lower, both of type char.
        var joinQuery2 =
            from lower in lowerCase
            where lower != 'x'
            from upper in upperCase
            select new { lower, upper };


        // Execute the queries.
        Console.WriteLine("Cross join:");
        // Rest the mouse pointer on joinQuery1 to verify its type.
        foreach (var pair in joinQuery1)
        {
            Console.WriteLine("{0} is matched to {1}", pair.upper, pair.lower);
        }

        Console.WriteLine("Filtered non-equijoin:");
        // Rest the mouse pointer over joinQuery2 to verify its type.
        foreach (var pair in joinQuery2)
        {
            Console.WriteLine("{0} is matched to {1}", pair.lower, pair.upper);
        }

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
/* Output:
        Cross join:
        A is matched to x
        A is matched to y
        A is matched to z
        B is matched to x
        B is matched to y
        B is matched to z
        C is matched to x
        C is matched to y
        C is matched to z
        Filtered non-equijoin:
        y is matched to A
        y is matched to B
        y is matched to C
        z is matched to A
        z is matched to B
        z is matched to C
        */