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В этом разделе вы узнаете о некоторых сложных запросах LINQ. Мы будем использовать следующие наборы данных о студентах и стандартные наборы данных для запросов.
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" }
};Пример: несколько операторов Select и Where
var studentNames = studentList.Where(s => s.Age > 18) .Select(s => s) .Where(st => st.StandardID > 0) .Select(s => s.StudentName);
Steve Ram
Следующий запрос возвращает множество анонимных объектов, имеющих только свойство 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));John Bill
Следующий запрос возвращает группы студентов, отсортированные по StandardID:
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));
}StandardID 0: Ron StandardID 1: John Steve StandardID 2: Bill Ram
Вывод включает в себя Ron без какого-либо StandardID, поэтому Ron принадлежит StandardID 0.
Чтобы удалить студентов без StandardID, используйте оператор where до оператора group:
var studentsGroupByStandard = from s in studentList
where s.StandardID > 0
group s by s.StandardID into sg
orderby sg.Key
select new { sg.Key, sg };StandardID 1: John Steve StandardID 2: Bill Ram
Используйте левую внешнюю связку (Left outer join), чтобы отображать студентов под каждой стандартом. Даже если у студентов нет присвоенного стандарта, показывается имя стандарта.
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));
}Standard 1: John Steve Standard 2: Bill Ram Standard 3:
In the following group by query example, we sort the groups and only select 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);
}John is in Standard 1 Steve is in Standard 1 Bill is in Standard 2 Ram is in Standard 2
The following query returns the student list in ascending order by StandardID and Age.
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));Student Name: Ron, Age: 21, StandardID: 0 Student Name: John, Age: 18, StandardID: 1 Student Name: Steve, Age: 21, StandardID: 1 Student Name: Bill, Age: 18, StandardID: 2 Student Name: Ram, Age: 20, StandardID: 2
var studentWithStandard = from s in studentList
join stad in standardList
on s.StandardID equals stad.StandardID
select new {
StudentName = s.StudentName,
StandardName = stad.StandardName
};
studentWithStandard.ToList().ForEach(s => Console.WriteLine("{0} is in {1}", s.StudentName, s.StandardName));John is in Standard 1 Steve is in Standard 1 Bill is in Standard 2 Ram is in Standard 2
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));
Steve