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