在本節(jié)中，您將學(xué)習(xí)一些復(fù)雜的LINQ查詢。我們將使用以下學(xué)生和標(biāo)準(zhǔn)集合進(jìn)行查詢。
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"}
};示例：多個(gè)Select和where運(yùn)算符
var studentNames = studentList.Where(s => s.Age > 18) .Select(s => s) .Where(st => st.StandardID > 0) .Select(s => s.StudentName);
Steve Ram
以下查詢返回僅具有StudentName屬性的匿名對象的Enumerable：
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
以下查詢返回按StandardID列出的學(xué)生組：
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
輸出包括沒有任何 StandardID 的 Ron，因此 Ron 屬于 StandardID 0。
要?jiǎng)h除沒有StandardID的學(xué)生，請?jiān)诮M運(yùn)算符之前使用where運(yùn)算符：
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
使用左外部聯(lián)接(Left outer join)顯示每個(gè)標(biāo)準(zhǔn)下的學(xué)生。即使沒有分配該標(biāo)準(zhǔn)的學(xué)生，也要顯示標(biāo)準(zhǔn)名稱。
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:
在下面的group by查詢示例中，我們對組進(jìn)行排序并只選擇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
以下查詢按StandardID和Age的升序返回學(xué)生列表。
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