LINQ 学习路程 -- 查询例子

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

IList<Standard> standardList = new List<Standard>() { 
    new Standard(){ StandardID = 1, StandardName="Standard 1"},
    new Standard(){ StandardID = 2, StandardName="Standard 2"},
    new Standard(){ StandardID = 3, StandardName="Standard 3"}
};

1.多个select和where操作

var studentNames = studentList.Where(s => s.Age > 18)
                              .Select(s => s)
                              .Where(st => st.StandardID > 0)
                              .Select(s => s.StudentName);

var teenStudentsName = from s in studentList
                       where s.age > 12 && s.age < 20
                       select new { StudentName = s.StudentName };

teenStudentsName.ToList().ForEach(s => Console.WriteLine(s.StudentName));

2.Group by

var studentsGroupByStandard = from s in studentList
                              group s by s.StandardID into sg
                              orderby sg.Key 
                                    select new { sg.Key, sg };


foreach (var group in studentsGroupByStandard)
{
    Console.WriteLine("StandardID {0}:", group.Key);
    
    group.sg.ToList().ForEach(st => Console.WriteLine(st.StudentName ));
}

left outer join

var studentsGroup = from stad in standardList
                    join s in studentList
                    on stad.StandardID equals s.StandardID
                        into sg
                        select new { 
                                        StandardName = stad.StandardName, 
                                        Students = sg 
                                    };

foreach (var group in studentsGroup)
{
    Console.WriteLine(group.StandardName);
    
    group.Students.ToList().ForEach(st => Console.WriteLine(st.StudentName));
}

var studentsWithStandard = from stad in standardList
                           join s in studentList
                           on stad.StandardID equals s.StandardID
                           into sg
                               from std_grp in sg 
                               orderby stad.StandardName, std_grp.StudentName 
                               select new { 
                                                StudentName = std_grp.StudentName, 
                                                StandardName = stad.StandardName 
                                };


foreach (var group in studentsWithStandard)
{
    Console.WriteLine("{0} is in {1}", group.StudentName, group.StandardName);
}

Sorting

var sortedStudents = from s in studentList
                        orderby s.StandardID, s.age
                        select new { 
                                StudentName = s.StudentName, 
                                Age = s.age, 
                                StandardID = s.StandardID };

sortedStudents.ToList().ForEach(s => Console.WriteLine("Student Name: {0}, Age: {1}, StandardID: {2}", s.StudentName, s.Age , s.StandardID));

inner join

var studentWithStandard = from s in studentList
                          join stad in standardList
                          on s.StandardID equals stad.StandardID 
                          select new { 
                                  StudentName = s.StudentName, 
                                  StandardName = stad.StandardName 
                              };

var nestedQueries = from s in studentList
                    where s.age > 18 && s.StandardID == 
                        (from std in standardList
                        where std.StandardName == "Standard 1"
                        select std.StandardID).FirstOrDefault()
                            select s;

nestedQueries.ToList().ForEach(s => Console.WriteLine(s.StudentName));