LINQ入门教程之各种标准查询操作符（一）

好久之前就想系统的学习下LINQ,好久之前……

本篇文章主要介绍LINQ等的标准查询操作符，内容取自《LINQ高级编程》，后续还会介绍LINQ to XML ,LINQ to SQL。

LINQ入门教程之各种标准查询操作符（二）

进入正题

（一）先介绍下标准查询操作符的概念

它是一系列方法的集合，如常见的Select 、Where、OrderBy等，这些方法作用于序列(集合)之上，序列是一个实现了IEnumerable<T>接口或IQueryable<T>接口的类型的对象。

IEnumerable和IQueryable的一个区别是:IEnumerable接口提供了向前遍历的功能。它不具备在各个数据项之间移动（向前移动除外）的能力。IQueryalbe可以使查询操作更具灵活性。

通俗的讲查询操作符是用好linq的基本，就像砍材要用的刀一样……

（二）另外再说下查询语法和方法语法的概念

查询语法即用查询表达式来编写查询。如代码：

1 int[] data={1,2,3,4};
2 IEnumerable<int> query = from da in data where da > 2 select da;

那方法语法呢如代码：

1 int[] data={1,2,3,4};
2 IEnumerable<int> query2 = data.Select(o => o).Where(d => d > 2);

LINQ在编译的时候，查询语法将会转为方法语法。相对而言，查询语法更容易阅读、理解和维护

（三）各种查询操作符汇总列表

标准查询操作符 (方法语法)	查询表达式（查询语法）
All(Of T)	N/A
Any	N/A
Average	N/A
Cast(Of T)	An explicit range of variables
Count	N/A
Distinct	N/A
GroupBy	group by
GroupJoin	join…in…on…into
Join	join…in…on…equals…
LongCount	N/A
Max	N/A
Min	N/A
OrderBy	order by
OrderByDescending	order by descending
Select	select
SelectMany	Multiple from clauses
Skip	N/A
SkipWhile	N/A
Sum	N/A
Take	N/A
TakeWhile	N/A
ThenBy	order by
ThenByDescending	order by descending
Where	where

备注：N/A (Not Applicable 不适用的)

(四) 下面主要通过代码来讲解各个标准查询操作符。

1．投影操作符 (Select SelectMany)

 1   #region  投影操作符 将序列中的元素转换为一个由开发人员定义的形式的操作
 2        /// <summary>
 3        /// 对单个序列或集合中的值进行投影
 4        /// </summary>
 5         static void SelectFunction()
 6         {
 7             DataContext contexts = new DataContext("Initial Catalog=AdventureWorks; Integrated Security=true");
 8             Table<Contact> contacts = contexts.GetTable<Contact>();
 9             //查询语法
10             var query = from c in contacts where c.FirstName  == "gong"  select new { c.FirstName };
11             //方法语法
12             query = contacts.Select(o => new { o.FirstName }).Where(c => c.FirstName == "gong");
13             foreach (var item in query)
14             {
15                 Console.WriteLine(item.FirstName);
16             }
17         }
18         //output:
19         //Gong
20         //Gong
21         class name{ public List<string> pets; public string FirstName;}
22         /// <summary>
23         /// selectMany 提供了将多个from子句组合起来的功能，它将每个对象的结果合并成单个序列
24         /// </summary>
25         static void SelectManyFunction( )
26         {
27           //List<name> list =new List<name>() { 
28           name[] list =new name[] { 
29               new name{ FirstName="Gwt",pets=new List<string>{"TEST"} }  ,
30               new name{ FirstName="Gwt1",pets=new List<string>{"TEST4","Test41"} }, 
31           };
32           //将所有List<string> 合并
33             IEnumerable<string> query = list.SelectMany(own => own.pets);
34           foreach (string item in query)
35           {
36               Console.Write(item +" ");
37           }
38           Console.WriteLine("");
39           //下面这个循环等于上面的seletmany方法
40             var query2 = list.Select(own => own.pets);
41           foreach (var item in query2)
42           {
43               foreach (var item2 in item)
44               {
45                   Console.Write(item2+" ");
46               }
47           }
48         }
49         //output:
50         // TEST TEST4 TEST41
51         // TEST TEST4 TEST41

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2．限制操作符 (Where)

 1     #region  限制操作符
 2         /// <summary>
 3         /// where操作符不启动查询的执行，当开始对对象进行枚举时查询才开始执行
 4         /// </summary>
 5         static void WhereFunction()
 6         {
 7             int[] numbers = { 1,2,3,4,5,6};
 8             IEnumerable<int> query = numbers.Select(o => o).Where(o => o > 3);
 9             foreach (int item in query)
10             {
11                 Console.Write(item+" ");
12             }
13         }
14         //output;
15         //4 5 6
16         #endregion

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3．排序操作符 (OrderBy OrderByDescending ThenBy ThenByDescending Reverse)

 1      #region  排序操作符 
 2         /// <summary>
 3         /// OrderBy/OrderByDescending     对应提供了按照升序/降序的方式对结果进行排序的功能
 4         /// ThenBy/ThenByDescending        实现按照次关键字对序列进行升序/降序排列
 5         /// </summary>
 6         static void OrderByFunction()
 7         {
 8             DataContext context = new DataContext(StaticData.ConnectSql);
 9             Table<Contact> contacts = context.GetTable<Contact>( );
10             IEnumerable<Contact> query = contacts.Select(o => o).OrderBy(o => o.FirstName).ThenBy(o => o.LastName);
11         
12             foreach (Contact item in query)
13             {
14                 Console.WriteLine(item.FirstName + " " + item.LastName);
15             }
16             Console.WriteLine();
17             query = contacts.Select(o => o).OrderByDescending(o => o.FirstName).ThenByDescending(o => o.LastName);
18             foreach (Contact item in query)
19             {
20                 Console.WriteLine(item.FirstName + " " + item.LastName);
21             }
22         }
23         //output:
24         //Gong tao
25         //Gong taos
26         //gong2 tao2
27         //gong3 tao3
28         //gong4 tao4
29         //gong5 tao5
30         //
31         //gong5 tao5
32         //gong4 tao4
33         //gong3 tao3
34         //gong2 tao2
35         //Gong taos
36         //Gong tao
37         /// <summary>
38         /// Reverse 把数据源的数据按相反的顺序返回
39         /// </summary>
40         static void ReverseFunction()
41         {
42             int[] numbers = { 1,2,3,4,5};
43             IEnumerable<int> query = numbers.Reverse();
44             foreach (int item in query)
45             {
46                 Console.Write(item+"  ");
47             }
48         }
49         //output:
50         //5 4 3 2 1
51         #endregion

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4．联接操作符 (Join GroupJoin)

 1   #region  联接操作符
 2         /// <summary>
 3         /// join 相当于T-SQL中的inner join 
 4         /// </summary>
 5         static void JoinFunction()
 6         {
 7             DataContext context = new DataContext(StaticData.ConnectSql);
 8             Table<Contact> contact = context.GetTable<Contact>();
 9             Table<Employee> employee = context.GetTable<Employee>();
10             //方法语法
11             var query = contact.Join(employee, con => con.ContactID, emp => emp.ContactID,
12                 (con, emp) => new { ContactName = con.FirstName, emp })
13                 .Where(con=>con.ContactName=="gong2");
14             //查询语法
15             query = from con in contact
16                     join emp in employee on con.ContactID equals emp.ContactID 
17                     where con.FirstName=="gong2"
18                      select new { ContactName = con.FirstName, emp } ; 
19             foreach (var item in query)
20             {
21                 Console.WriteLine(item.ContactName + "  " + item.emp.EmployeeID);
22             }
23         }
24         //output:
25         //gong2  23
26         //gong2  24
27         //gong2  25
28         //gong2  26
29         class Team
30         {
31             public string TeamName; 
32         }
33         class Rider
34         {
35             public string Name;
36             public string TeamName;
37         }
38         /// <summary>
39         /// GroupJoin  
40         /// </summary>
41         static void GroupJoinFunction()
42         {
43             List<Team> teams = new List<Team>{new Team{ TeamName="t1"}
44             ,new Team{TeamName="t2"}
45            ,new Team{TeamName="t3"}
46            };
47             List<Rider> riders = new List<Rider> { 
48             new Rider{Name="r1",TeamName="t1"},
49             new Rider{Name="r2",TeamName="t1"},
50             new Rider{Name="r3",TeamName="t2"},
51             new Rider{Name="r4",TeamName="t2"},
52             new Rider{Name="r5",TeamName="t3"},
53             new Rider{Name="r6",TeamName="t3"},
54             };
55             //方法语法
56             var query = teams.GroupJoin(riders, t => t.TeamName, r => r.TeamName,
57                 (t, r) => new { teamname = t.TeamName, ridernames = r.Select(o => o.Name) });
58             foreach (var item in query)
59             {
60                 Console.WriteLine(item.teamname);
61                 foreach (var item2 in item.ridernames)
62                 {
63                     Console.WriteLine("     "+item2);
64                 }
65             }
66         }
67         //output:
68         //t1
69         //    r1
70         //    r2
71         //t2
72         //    r3
73         //    r4
74         //t3
75         //    r5
76         //    r6
77         #endregion

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5．分组操作符 (GroupBy)

 1 #region  分组操作符  根据一个特定的值(选择器) 将序列中的值或元素进行分组
 2         /// <summary>
 3         ///
 4         /// </summary>
 5         static void GroupByFunction()
 6         {
 7             DataContext context = new DataContext(StaticData.ConnectSql);
 8             Table<SalesOrderHeader> orders = context.GetTable<SalesOrderHeader>();
 9             //按照每个销售人员对订单进行分组
10             var query = orders.Where(o => o.SalesPersonID > 0).GroupBy(order => order.SalesPersonID, order => order.CustomerID);
11 
12             foreach (var item in query)
13             {
14                 Console.WriteLine(item.Key);
15                 foreach (var item2 in item)
16                 {
17                     Console.WriteLine("  " + item2);
18                 }
19             }
20         }
21         #endregion

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6．串联操作符 (Concat)

 1  #region  串联操作符
 2         /// <summary>
 3         /// 串联操作符 Concat 【将两个集合合并成一个集合】
 4         /// </summary>
 5         static void ConcatFunction( )
 6         {
 7             DataContext context = new DataContext(StaticData.ConnectSql);
 8             Table<Contact> contacts = context.GetTable<Contact>( );
 9             Table<SalesOrderHeader> orders = context.GetTable<SalesOrderHeader>();
10             //将Contact表的Firstname和SalesOrderHeader的CustomerID串联起来
11             var query = contacts.Select(con => con.FirstName).Concat(orders.Select(order => order.CustomerID.ToString()));
12             foreach (var item in query)
13             {
14                 Console.WriteLine(item.ToString());
15             }
16           }
17         #endregion

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LINQ入门教程之各种标准查询操作符（二）

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