LINQ 101——分区、Join、聚合

一、Partitioning 分区

Take

例1：取前3个数

static void Linq1()
{
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    var first3Numbers = numbers.Take(3);
    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 = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    var skip3Numbers = numbers.Skip(3);
    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-1) * 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