（转）Linq-查询操作符之Select、Where、OrderBy、OrderByDescending

介绍

Select - 选择；延迟

Where - 查询；延迟

OrderBy - 按指定表达式对集合正序排序；延迟

OrderByDescending - 按指定表达式对集合倒序排序；延迟

GroupBy - 分组；延迟

Join - 查询；延迟

GroupJoin - 分组Join查询；延迟

以上查询操作符所对应的查询语法

示例

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Xml.Linq; 

using System.Collections.Generic;
using DAL; 

public partial class LINQ_Summary : System.Web.UI.Page
{
        NorthwindDataContext _ctx = new NorthwindDataContext();
        string[] _ary = null; 

        protected void Page_Load(object sender, EventArgs e)
        {
                _ary = new string[] { "asp.net", "csharp", "xhtml", "css", "javascript",
                        "wcf", "wpf", "silverlight", "linq", "wf",
                        "sqlserver", "asp.net ajax", "ssis", "ssas", "ssrs" }; 

                // Select - Select选择；延迟
                Summary_Select(); 

                // Where - Where查询；延迟
                Summary_Where(); 

                // OrderBy - 按指定表达式对集合正序排序；延迟
                // OrderByDescending - 按指定表达式对集合倒序排序；延迟
                Summary_OrderBy_OrderByDescending(); 

                // GroupBy - 分组；延迟
                Summary_GroupBy(); 

                // Join - Join查询；延迟
                Summary_Join(); 

                // GroupJoin - 分组Join查询；延迟
                Summary_GroupJoin();
        }
}

/// <summary>
/// Select - 选择；延迟
/// </summary>
void Summary_Select()
{
        // 使用Select查询操作符
        var categories = _ctx.Categories.Select(
                c => new { CategoryName = "类别名称：" + c.CategoryName }); 

        foreach (var c in categories)
        {
                result.InnerHtml += c.CategoryName + "<br />";
        }
        result.InnerHtml += "<br />"; 

        // 与上面的Select查询操作符相对应的查询语法
        var categories2 = from c in _ctx.Categories
                                            select new { CategoryName = "类别名称：" + c.CategoryName }; 

        foreach (var c in categories2)
        {
                result.InnerHtml += c.CategoryName + "<br />";
        }
        result.InnerHtml += "<br />";
}

运行结果

类别名称：Beverages

类别名称：Condiments

类别名称：Confections

类别名称：Dairy Products

类别名称：Grains/Cereals

类别名称：Meat/Poultry

类别名称：Produce

类别名称：Seafood

/// <summary>
/// Where - 查询；延迟
/// </summary>
void Summary_Where()
{
        // 使用Where查询操作符
        var ary = _ary.Where(a => a.StartsWith("w") && a.EndsWith("f")); 

        foreach (string s in ary)
        {
                result.InnerHtml += s + "<br />";
        }
        result.InnerHtml += "<br />"; 

        // 与上面的Where查询操作符相对应的查询语法
        var ary2 = from a in _ary
                             where a.StartsWith("w") && a.EndsWith("f")
                             select a; 

        foreach (string s in ary2)
        {
                result.InnerHtml += s + "<br />";
        }
        result.InnerHtml += "<br />";
}

运行结果

wcf

wpf

wf

/// <summary>
/// OrderBy - 按指定表达式对集合正序排序；延迟
/// OrderByDescending - 按指定表达式对集合倒序排序；延迟
/// </summary>
void Summary_OrderBy_OrderByDescending()
{
        // 使用OrderBy查询操作符
        var ary = (from a in _ary
                             select a).OrderBy(a => a.Length); // OrderByDescending与OrderBy用法相同 

        foreach (string s in ary)
        {
                result.InnerHtml += s + "<br />";
        }
        result.InnerHtml += "<br />"; 

        // 与上面的OrderBy查询操作符相对应的查询语法
        var ary2 = from a in _ary
                             orderby a.Length ascending // orderby xxx descending与orderby xxx ascending用法相同
                             select a; 

        foreach (string s in ary2)
        {
                result.InnerHtml += s + "<br />";
        }
        result.InnerHtml += "<br />";
}

运行结果

wf

css

wcf

wpf

linq

ssis

ssas

ssrs

xhtml

csharp

asp.net

sqlserver

javascript

silverlight

asp.net ajax

/// <summary>
/// GroupBy - 分组；延迟
/// </summary>
void Summary_GroupBy()
{
        // 使用GroupBy查询操作符
        var list = (from a in _ary
                                select a).GroupBy(a => a.Length).Select(
                                        g => new { Group = g.Key, Member = g }); 

        foreach (var g in list)
        {
                result.InnerHtml += g.Group + "个字符：<br />"; 

                foreach (string s in g.Member)
                {
                        result.InnerHtml += "--" + s + "<br />";
                }
        }
        result.InnerHtml += "<br />"; 

        // 与上面的GroupBy查询操作符相对应的查询语法
        var list2 = from a in _ary
                                group a by a.Length into g
                                select new { Group = g.Key, Member = g }; 

        foreach (var g in list2)
        {
                result.InnerHtml += g.Group + "个字符：<br />"; 

                foreach (string s in g.Member)
                {
                        result.InnerHtml += "--" + s + "<br />";
                }
        }
        result.InnerHtml += "<br />";
}

运行结果

7个字符：

--asp.net

6个字符：

--csharp

5个字符：

--xhtml

3个字符：

--css

--wcf

--wpf

10个字符：

--javascript

11个字符：

--silverlight

4个字符：

--linq

--ssis

--ssas

--ssrs

2个字符：

--wf

9个字符：

--sqlserver

12个字符：

--asp.net ajax

/// <summary>
/// Join - 查询；延迟
/// </summary>
void Summary_Join()
{
        // 使用Join查询操作符
        var products = _ctx.Products.Join(
                _ctx.Categories,
                p => p.CategoryID,
                c => c.CategoryID,
                (p, c) => new { c.CategoryName, p.ProductName }).Take(5); 

        foreach (var p in products)
        {
                result.InnerHtml += p.CategoryName + " - " + p.ProductName + "<br />";
        }
        result.InnerHtml += "<br />"; 

        // 与上面的Join查询操作符相对应的查询语法
        var products2 = (from p in _ctx.Products
                                         join c in _ctx.Categories
                                         on p.CategoryID equals c.CategoryID
                                         select new { c.CategoryName, p.ProductName }).Take(5); 

        foreach (var p in products2)
        {
                result.InnerHtml += p.CategoryName + " - " + p.ProductName + "<br />";
        }
        result.InnerHtml += "<br />";
}

运行结果

Beverages - Chai

Beverages - Chang

Condiments - Aniseed Syrup

Condiments - Chef Anton‘s Cajun Seasoning

Condiments - Chef Anton‘s Gumbo Mix

/// <summary>
/// GroupJoin - 分组Join查询；延迟
/// </summary>
void Summary_GroupJoin()
{
        // 使用GroupJoin查询操作符
        var products = _ctx.Categories.GroupJoin(
                _ctx.Products,
                c => c.CategoryID,
                p => p.CategoryID,
                (p, g) => new { p.CategoryName, ProductCount = g.Count() }); 

        foreach (var g in products)
        {
                result.InnerHtml += g.CategoryName + "：" + g.ProductCount + "<br />"; 

        }
        result.InnerHtml += "<br />"; 

        // 与上面的GroupJoin查询操作符相对应的查询语法
        var products2 = from c in _ctx.Categories
                                        join p in _ctx.Products on c.CategoryID equals p.CategoryID into g
                                        select new { CategoryName = c.CategoryName, ProductCount = g.Count() }; 

        foreach (var g in products2)
        {
                result.InnerHtml += g.CategoryName + "：" + g.ProductCount + "<br />"; 

        }
        result.InnerHtml += "<br />";
}

运行结果

Beverages：12

Condiments：12

Confections：13

Dairy Products：10

Grains/Cereals：7

Meat/Poultry：6

Produce：5

Seafood：12

原文链接：

http://www.cnblogs.com/qiuweiguo/archive/2011/08/03/2126600.html

原文地址：https://www.cnblogs.com/MingsonZheng/p/9404703.html