LINQ 学习路程 -- 查询语法 LINQ Query Syntax

1.查询语法 

Query Syntax:

from <range variable> in <IEnumerable<T> or IQueryable<T> Collection>

<Standard Query Operators> <lambda expression>

<select or groupBy operator> <result formation>

　　

// string collection
IList<string> stringList = new List<string>() { 
    "C# Tutorials",
    "VB.NET Tutorials",
    "Learn C++",
    "MVC Tutorials" ,
    "Java" 
};

// LINQ Query Syntax
var result = from s in stringList
            where s.Contains("Tutorials") 
            select s;

查询语法以From开头，后面紧跟着Range veriable变量，From从句像这样的结构"From rangeVariableName in IEnumerablecollection"，意思是从集合的每个对象中获取，它有点像foreach循环，

 foreach(Student s in studentList)

在From从句之后，我们可以使用不同的标准查询运算符来过滤、分类和联合集合里面的元素，大概有50个标准查询操作

一般以Select 或Group从句结尾，Select从句用来构建数据，你可以查询全部对象或者它的几个属性。

IList<Student> studentList = new List<Student>>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

// LINQ Query Syntax to find out teenager students
var teenAgerStudent = from s in studentList
                      where s.Age > 12 && s.Age < 20
                      select s;
                

LINQ Method Syntax

// string collection
IList<string> stringList = new List<string>() { 
    "C# Tutorials",
    "VB.NET Tutorials",
    "Learn C++",
    "MVC Tutorials" ,
    "Java" 
};

// LINQ Query Syntax
var result = stringList.Where(s => s.Contains("Tutorials"));

 

            
// Student collection
IList<Student> studentList = new List<Student>() { 
        new Student() { StudentID = 1, StudentName = "John", Age = 13} ,
        new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,
        new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
        new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,
        new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 
    };

// LINQ Method Syntax to find out teenager students
var teenAgerStudents = studentList.Where(s => s.Age > 12 && s.Age < 20)
                                  .ToList<Student>();

Standard Query Operators:

Standard Query Operators in Query Syntax:

 

Standard Query Operators in Method Syntax:

 

标准查询操作根据功能分类 

Classification	Standard Query Operators
Filtering（筛选）	Where, OfType
Sorting（排序）	OrderBy, OrderByDescending, ThenBy, ThenByDescending, Reverse
Grouping（分组）	GroupBy, ToLookup
Join（连接）	GroupJoin, Join
Projection（投影）	Select, SelectMany
Aggregation（聚集）	Aggregate, Average, Count, LongCount, Max, Min, Sum
Quantifiers（量词）	All, Any, Contains
Elements（元素）	ElementAt, ElementAtOrDefault, First, FirstOrDefault, Last, LastOrDefault, Single, SingleOrDefault
Set（集合）	Distinct, Except, Intersect, Union
Partitioning（分割）	Skip, SkipWhile, Take, TakeWhile
Concatenation（连接）	Concat
Equality	SequenceEqual
Generation	DefaultEmpty, Empty, Range, Repeat
Conversion	AsEnumerable, AsQueryable, Cast, ToArray, ToDictionary, ToList