- 相關(guān)推薦
c#關(guān)鍵字查詢之select 子句運(yùn)用
引導(dǎo)語(yǔ)：在計(jì)算機(jī)中，select語(yǔ)句是最常用的數(shù)據(jù)查詢語(yǔ)句。以下是小編整理的c#關(guān)鍵字查詢之select 子句運(yùn)用，歡迎參考閱讀!
在查詢表達(dá)式中，select 子句可以指定將在執(zhí)行查詢時(shí)產(chǎn)生的值的類型。該子句的結(jié)果將基于前面所有子句的計(jì)算結(jié)果以及 select 子句本身中的所有表達(dá)式。查詢表達(dá)式必須以 select 子句或 group 子句結(jié)束。
下面的示例演示了查詢表達(dá)式中的簡(jiǎn)單 select 子句。
C#
class SelectSample1
{
static void Main()
{
//Create the data source
List<int> Scores = new List<int>() { 97, 92, 81, 60 };
// Create the query.
IEnumerable<int> queryHighScores =
from score in Scores
where score > 80
select score;
// Execute the query.
foreach (int i in queryHighScores)
{
Console.Write(i + " ");
}
}
}
//Output: 97 92 81
select 子句產(chǎn)生的序列的類型決定了查詢變量 queryHighScores 的類型。在最簡(jiǎn)單的情況下，select 子句僅指定范圍變量。這會(huì)使返回的序列包含與數(shù)據(jù)源具有相同類型的元素。有關(guān)更多信息，請(qǐng)參見(jiàn)查詢操作中的類型關(guān)系 (LINQ)。不過(guò)，select 子句還提供了一種功能強(qiáng)大的機(jī)制，可用于將源數(shù)據(jù)轉(zhuǎn)換（或投影）為新類型。有關(guān)更多信息，請(qǐng)參見(jiàn)使用 LINQ 進(jìn)行數(shù)據(jù)轉(zhuǎn)換。
示例
下面的示例演示了 select 子句可能采用的所有不同形式。在每個(gè)查詢中，請(qǐng)注意 select 子句和查詢變量（studentQuery1、studentQuery2 等）的類型之間的關(guān)系。
C#
class SelectSample2
{
// Define some classes
public class Student
{
public string First { get; set; }
public string Last { get; set; }
public int ID { get; set; }
public List<int> Scores;
public ContactInfo GetContactInfo(SelectSample2 app, int id)
{
ContactInfo cInfo =
(from ci in app.contactList
where ci.ID == id
select ci)
.FirstOrDefault();
return cInfo;
}
public override string ToString()
{
return First + " " + Last + ":" + ID;
}
}
public class ContactInfo
{
public int ID { get; set; }
public string Email { get; set; }
public string Phone { get; set; }
public override string ToString() { return Email + "," + Phone; }
}
public class ScoreInfo
{
public double Average { get; set; }
public int ID { get; set; }
}
// The primary data source
List<Student> students = new List<Student>()
{
new Student {First="Svetlana", Last="Omelchenko", ID=111, Scores= new List<int>() {97, 92, 81, 60}},
new Student {First="Claire", Last="O'Donnell", ID=112, Scores= new List<int>() {75, 84, 91, 39}},
new Student {First="Sven", Last="Mortensen", ID=113, Scores= new List<int>() {88, 94, 65, 91}},
new Student {First="Cesar", Last="Garcia", ID=114, Scores= new List<int>() {97, 89, 85, 82}},
};
// Separate data source for contact info.
List<ContactInfo> contactList = new List<ContactInfo>()
{
new ContactInfo {ID=111, Email="SvetlanO@Contoso.com", Phone="206-555-0108"},
new ContactInfo {ID=112, Email="ClaireO@Contoso.com", Phone="206-555-0298"},
new ContactInfo {ID=113, Email="SvenMort@Contoso.com", Phone="206-555-1130"},
new ContactInfo {ID=114, Email="CesarGar@Contoso.com", Phone="206-555-0521"}
};
static void Main(string[] args)
{
SelectSample2 app = new SelectSample2();
// Produce a filtered sequence of unmodified Students.
IEnumerable<Student> studentQuery1 =
from student in app.students
where student.ID > 111
select student;
Console.WriteLine("Query1: select range_variable");
foreach (Student s in studentQuery1)
{
Console.WriteLine(s.ToString());
}
// Produce a filtered sequence of elements that contain
// only one property of each Student.
IEnumerable<String> studentQuery2 =
from student in app.students
where student.ID > 111
select student.Last;
Console.WriteLine(" studentQuery2: select range_variable.Property");
foreach (string s in studentQuery2)
{
Console.WriteLine(s);
}
// Produce a filtered sequence of objects created by
// a method call on each Student.
IEnumerable<ContactInfo> studentQuery3 =
from student in app.students
where student.ID > 111
select student.GetContactInfo(app, student.ID);
Console.WriteLine(" studentQuery3: select range_variable.Method");
foreach (ContactInfo ci in studentQuery3)
{
Console.WriteLine(ci.ToString());
}
// Produce a filtered sequence of ints from
// the internal array inside each Student.
IEnumerable<int> studentQuery4 =
from student in app.students
where student.ID > 111
select student.Scores[0];
Console.WriteLine(" studentQuery4: select range_variable[index]");
foreach (int i in studentQuery4)
{
Console.WriteLine("First score = {0}", i);
}
// Produce a filtered sequence of doubles
// that are the result of an expression.
IEnumerable<double> studentQuery5 =
from student in app.students
where student.ID > 111
select student.Scores[0] * 1.1;
Console.WriteLine(" studentQuery5: select expression");
foreach (double d in studentQuery5)
{
Console.WriteLine("Adjusted first score = {0}", d);
}
// Produce a filtered sequence of doubles that are
// the result of a method call.
IEnumerable<double> studentQuery6 =
from student in app.students
where student.ID > 111
select student.Scores.Average();
Console.WriteLine(" studentQuery6: select expression2");
foreach (double d in studentQuery6)
{
Console.WriteLine("Average = {0}", d);
}
// Produce a filtered sequence of anonymous types
// that contain only two properties from each Student.
var studentQuery7 =
from student in app.students
where student.ID > 111
select new { student.First, student.Last };
Console.WriteLine(" studentQuery7: select new anonymous type");
foreach (var item in studentQuery7)
{
Console.WriteLine("{0}, {1}", item.Last, item.First);
}
// Produce a filtered sequence of named objects that contain
// a method return value and a property from each Student.
// Use named types if you need to pass the query variable
// across a method boundary.
IEnumerable<ScoreInfo> studentQuery8 =
from student in app.students
where student.ID > 111
select new ScoreInfo
{
Average = student.Scores.Average(),
ID = student.ID
};
Console.WriteLine(" studentQuery8: select new named type");
foreach (ScoreInfo si in studentQuery8)
{
Console.WriteLine("ID = {0}, Average = {1}", si.ID, si.Average);
}
// Produce a filtered sequence of students who appear on a contact list
// and whose average is greater than 85.
IEnumerable<ContactInfo> studentQuery9 =
from student in app.students
where student.Scores.Average() > 85
join ci in app.contactList on student.ID equals ci.ID
select ci;
Console.WriteLine(" studentQuery9: select result of join clause");
foreach (ContactInfo ci in studentQuery9)
{
Console.WriteLine("ID = {0}, Email = {1}", ci.ID, ci.Email);
}
// Keep the console window open in debug mode
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
/* Output
Query1: select range_variable
Claire O'Donnell:112
Sven Mortensen:113
Cesar Garcia:114
studentQuery2: select range_variable.Property
O'Donnell
Mortensen
Garcia
studentQuery3: select range_variable.Method
ClaireO@Contoso.com,206-555-0298
SvenMort@Contoso.com,206-555-1130
CesarGar@Contoso.com,206-555-0521
studentQuery4: select range_variable[index]
First score = 75
First score = 88
First score = 97
studentQuery5: select expression
Adjusted first score = 82.5
Adjusted first score = 96.8
Adjusted first score = 106.7
studentQuery6: select expression2
Average = 72.25
Average = 84.5
Average = 88.25
studentQuery7: select new anonymous type
O'Donnell, Claire
Mortensen, Sven
Garcia, Cesar
studentQuery8: select new named type
ID = 112, Average = 72.25
ID = 113, Average = 84.5
ID = 114, Average = 88.25
studentQuery9: select result of join clause
ID = 114, Email = CesarGar@Contoso.com
*/
如上一個(gè)示例中的 studentQuery8 所示，您有時(shí)可能希望所返回序列中的元素僅包含源元素的屬性子集。通過(guò)使返回的序列盡可能地小一些，可以降低內(nèi)存需求，并提高查詢的執(zhí)行速度。通過(guò)在 select 子句中創(chuàng)建一個(gè)匿名類型，并且借助于對(duì)象初始值設(shè)定項(xiàng)用源元素中的適當(dāng)屬性對(duì)該匿名類型進(jìn)行初始化，可以達(dá)到此目的。
【c#關(guān)鍵字查詢之select 子句運(yùn)用】相關(guān)文章：
C#數(shù)據(jù)結(jié)構(gòu)之循環(huán)鏈表的實(shí)例代碼10-22
職稱英語(yǔ)成績(jī)查詢之合格標(biāo)準(zhǔn)06-02
淺談C#語(yǔ)言的特點(diǎn)11-01
C語(yǔ)言關(guān)鍵字08-31
關(guān)鍵字register分析07-24
Java編程中this關(guān)鍵字與super關(guān)鍵字的使用方法08-23
PHP中this關(guān)鍵字06-08