LINQ 101——分区、Join、聚合

一、Partitioning 分区

Take

例1：取前3个数

 static void Linq1()
 {
     int[] numbers = { , , , , , , , , ,  };
     var first3Numbers = numbers.Take();
     Console.WriteLine("前3个数:");
     foreach (var n in first3Numbers)
     {
         Console.WriteLine(n);
     }
 }

Skip

例2：跳过前3个数

 static void Linq2()
 {
     int[] numbers = { , , , , , , , , ,  };
     var skip3Numbers = numbers.Skip();
     Console.WriteLine("除去前3个数后数字:");
     foreach (var n in skip3Numbers)
     {
         Console.WriteLine(n);
     }
 }

Take + Skip 实现分页

 int currentPageIndex;　　// 当前页号
 int pageSize;　　　　　　// 每页有多少条记录
 source 是总数据源集合

 则用LINQ分页的代码为

 var query = source.Skip((currentPageIndex-) * pageSize).Take(pageSize);

二、Join

例1： Cross Join

 static void Linq3()
 {
     string[] cates = {"酒类","烟类","肉类" };
     var products = Product.GetDefaultData();

     // CategoryName 没有与 cate 匹配的将被剔除
     var query = from c in cates
                 join p in products on c equals p.CategoryName
                 select new { ShowName = c + " : " + p.ProductName };

     foreach (var item in query)
     {
         Console.WriteLine(item.ShowName);
     }
 }

 public class Product
 {
     public string CategoryName { get; set; }
     public string ProductName { get; set; }

     public static List<Product> GetDefaultData()
     {
         return new List<Product>
         {
             new Product{ CategoryName="烟类", ProductName="中华"},
             new Product{ CategoryName="酒类", ProductName="白酒"},
             new Product{ CategoryName="酒类", ProductName="红酒"},
             new Product{ CategoryName="肉类", ProductName="猪肉"},
             new Product{ CategoryName="肉类", ProductName="牛肉"},
             new Product{ CategoryName="零食", ProductName="饼干"},
             new Product{ ProductName="暂未分类产品1"},
             new Product{ ProductName="暂未分类产品2"},
         };
     }
 }

例2: Group Join

 static void Linq4()
 {
     string[] cates = { "酒类", "烟类", "肉类" };
     var products = Product.GetDefaultData();

     var query = from c in cates
                 join p in products on c equals p.CategoryName
                 into tgroup
                 select new { Name = c, Group = tgroup };

     foreach (var item in query)
     {
         Console.Write("{0}: ",item.Name);
         foreach (var g in item.Group)
         {
             Console.Write("{0} ",g.ProductName);
         }
         Console.WriteLine();
     }
 }

例3：Cross Join + Group Join

分组后再查询符合条件的

 static void Linq5()
 {
     string[] cates = { "酒类", "烟类", "肉类" };
     var products = Product.GetDefaultData();

     var query = from c in cates
                 join p in products on c equals p.CategoryName
                 into tgroup
                 from g in tgroup
                 where g.ProductName.Contains("牛腩")  // 仅要产品名字含有牛腩的分组
                 select new { Name = c, Group = tgroup };

     foreach (var item in query)
     {
         Console.Write("{0}: ", item.Name);
         foreach (var g in item.Group)
         {
             Console.Write("{0} ", g.ProductName);
         }
         Console.WriteLine();
     }
 }

例4 Left Outer Join

 static void Linq6()
 {
     string[] cates = { "酒类", "烟类", "肉类" ,"调料类"};
     var products = Product.GetDefaultData();

     var query = from c in cates
                 join p in products on c equals p.CategoryName
                 into tgroup
                 from g in tgroup.DefaultIfEmpty()
                 select new { Name = c, ProductName = g==null ? "该分组下没有内容":g.ProductName };

     foreach (var item in query)
     {
         Console.WriteLine("{0} {1}",item.Name,item.ProductName);
     }
 }

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