LINQ查询操作符之Select、Where、OrderBy、OrderByDescending、GroupBy、Join、GroupJoin及其对应的查询语法

介绍
    ·Select - Select选择；延迟
    ·Where - 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 - 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
Summary.aspx.cs

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();
    }
}

　　Select - Select选择；延迟

/// <summary>
    /// Select - 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

Where - Where查询；延迟

  /// <summary>
    /// Where - 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

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 - 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
　

GroupBy - 分组；延迟

/// <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

Join - Join查询；延迟

 /// <summary>
    /// Join - 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

GroupJoin - 分组Join查询；延迟

/// <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