LINQ 101——分区、Join、聚合

一、Partitioning 分区

Take

例1：取前3个数

 1 static void Linq1()
 2 {
 3     int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
 4     var first3Numbers = numbers.Take(3);
 5     Console.WriteLine("前3个数:");
 6     foreach (var n in first3Numbers)
 7     {
 8         Console.WriteLine(n);
 9     }
10 }

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Skip

例2：跳过前3个数

 1 static void Linq2()
 2 {
 3     int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
 4     var skip3Numbers = numbers.Skip(3);
 5     Console.WriteLine("除去前3个数后数字:");
 6     foreach (var n in skip3Numbers)
 7     {
 8         Console.WriteLine(n);
 9     }
10 }

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Take + Skip 实现分页

1 int currentPageIndex;　　// 当前页号
2 int pageSize;　　　　　　// 每页有多少条记录
3 source 是总数据源集合
4 
5 则用LINQ分页的代码为
6 
7 var query = source.Skip((currentPageIndex-1) * pageSize).Take(pageSize);

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二、Join

例1： Cross Join

 1 static void Linq3()
 2 {
 3     string[] cates = {"酒类","烟类","肉类" };
 4     var products = Product.GetDefaultData();
 5 
 6     // CategoryName 没有与 cate 匹配的将被剔除
 7     var query = from c in cates
 8                 join p in products on c equals p.CategoryName
 9                 select new { ShowName = c + " : " + p.ProductName };
10 
11     foreach (var item in query)
12     {
13         Console.WriteLine(item.ShowName);
14     }
15 }
16 
17 
18 public class Product
19 {
20     public string CategoryName { get; set; }
21     public string ProductName { get; set; }
22 
23     public static List<Product> GetDefaultData()
24     {
25         return new List<Product>
26         {
27             new Product{ CategoryName="烟类", ProductName="中华"},
28             new Product{ CategoryName="酒类", ProductName="白酒"},
29             new Product{ CategoryName="酒类", ProductName="红酒"},
30             new Product{ CategoryName="肉类", ProductName="猪肉"},
31             new Product{ CategoryName="肉类", ProductName="牛肉"},
32             new Product{ CategoryName="零食", ProductName="饼干"},
33             new Product{ ProductName="暂未分类产品1"},
34             new Product{ ProductName="暂未分类产品2"},
35         };
36     }
37 }

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例2: Group Join

 1 static void Linq4()
 2 {
 3     string[] cates = { "酒类", "烟类", "肉类" };
 4     var products = Product.GetDefaultData();
 5 
 6     var query = from c in cates
 7                 join p in products on c equals p.CategoryName
 8                 into tgroup
 9                 select new { Name = c, Group = tgroup };
10 
11     foreach (var item in query)
12     {
13         Console.Write("{0}: ",item.Name);
14         foreach (var g in item.Group)
15         {
16             Console.Write("{0} ",g.ProductName);
17         }
18         Console.WriteLine();
19     }
20 }

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例3：Cross Join + Group Join

分组后再查询符合条件的

 1 static void Linq5()
 2 {
 3     string[] cates = { "酒类", "烟类", "肉类" };
 4     var products = Product.GetDefaultData();
 5 
 6     var query = from c in cates
 7                 join p in products on c equals p.CategoryName
 8                 into tgroup
 9                 from g in tgroup
10                 where g.ProductName.Contains("牛腩")  // 仅要产品名字含有牛腩的分组
11                 select new { Name = c, Group = tgroup };
12 
13     foreach (var item in query)
14     {
15         Console.Write("{0}: ", item.Name);
16         foreach (var g in item.Group)
17         {
18             Console.Write("{0} ", g.ProductName);
19         }
20         Console.WriteLine();
21     }
22 }

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例4 Left Outer Join

 1 static void Linq6()
 2 {
 3     string[] cates = { "酒类", "烟类", "肉类" ,"调料类"};
 4     var products = Product.GetDefaultData();
 5 
 6     var query = from c in cates
 7                 join p in products on c equals p.CategoryName
 8                 into tgroup
 9                 from g in tgroup.DefaultIfEmpty()
10                 select new { Name = c, ProductName = g==null ? "该分组下没有内容":g.ProductName };
11             
12     foreach (var item in query)
13     {
14         Console.WriteLine("{0} {1}",item.Name,item.ProductName);
15     }
16 }

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三、聚合

Count();

Count(n => 条件); 后面都类似

Sum();

Min()

Max()

Average()

本文代码：http://files.cnblogs.com/Aphasia/ConsoleApplication2.rar