ObjectQuery 类支持对 实体数据模型 (EDM) 执行 LINQ to Entities 和 Entity SQL 查询。ObjectQuery 还实现了一组查询生成器方法,这些方法可用于按顺序构造等效于 Entity SQL 的查询命令。下面是 ObjectQuery 的查询生成器方法以及等效的 Entity SQL 语句:
Distinct,Except,GroupBy,Intersect,OfType,OrderBy,Select,SelectValue,Skip,Top,Union,UnionAll,Where
每个查询生成器方法返回 ObjectQuery 的一个新实例。使用这些方法可以构造查询,而查询的结果集基于前面 ObjectQuery 实例序列的操作。下面来看具体的代码片断:

> Execute方法:

using (var edm = new NorthwindEntities())
{
string esql = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query = edm.CreateQuery<Customers>(esql);
ObjectResult<Customers> results = query.Execute(MergeOption.NoTracking);
Assert.AreEqual(results.Count(), );
foreach (Customers c in query)
Console.WriteLine(c.CustomerID);
}

其中需要说明的是: MergeOption这个枚举类型的参数项,MergeOption有四种值分别是:

> AppendOnly: 只追加新实体,不修改以前获取的现有实体。这是默认行为。

> OverwriteChanges: 将 ObjectStateEntry 中的当前值替换为存储区中的值。这将使用服务器上的数据重写在本地所做的更改。

> PreserveChanges: 将替换原始值,而不修改当前值。这对于在发生开放式并发异常之后强制成功保存本地值非常有用。

> NoTracking: 将不修改 ObjectStateManager,不会获取与其他对象相关联的关系,可以改善性能。

> GetResultType方法:返回查询结果的类型信息.例如:

using (var edm = new NorthwindEntities())
{
string esql = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query = edm.CreateQuery<Customers>(esql);
Console.WriteLine(query.GetResultType().ToString());
//输出结果为:
//NorthWindModel.Customers
}

> ToTraceString方法:获取当前执行的SQL语句。

> Where

实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql = "select value c from NorthwindEntities.Customers as c ";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql);
//使用ObjectParameter的写法
query1 = query1.Where("it.CustomerId=@customerid");
query1.Parameters.Add(new ObjectParameter("customerid", "ALFKI"));
//也可以这样写
//ObjectQuery<Customers> query2 = edm.Customers.Where("it.CustomerID='ALFKI'");
foreach (var c in query1)
Console.WriteLine(c.CustomerID);
//显示查询执行的SQL语句
Console.WriteLine(query1.ToTraceString()); }

> First/ FirstOrDefault

实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query = edm.CreateQuery<Customers>(esql);
Customers c1 = query.First();
Customers c2 = query.FirstOrDefault();
Console.WriteLine(c1.CustomerID);
Assert.IsNotNull(c2);
Console.WriteLine(c2.CustomerID);
}

> Distinct

实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql = "select value c.City from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<string> query = edm.CreateQuery<string>(esql);
query = query.Distinct();
foreach (string c in query)
{
Console.WriteLine("City {0}", c);
}
}

> Except:返回两个查询的差集。实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql1 = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql1);
string esql2 = "select value c from NorthwindEntities.Customers as c where c.Country='UK' order by c.CustomerID limit 10";
ObjectQuery<Customers> query2 = edm.CreateQuery<Customers>(esql2);
query1 = query1.Except(query2);
foreach (Customers c in query1)
{
Console.WriteLine(c.Country);
//输出:UK
}
}

> Intersect:返回两个查询的交集。实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql1 = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql1);
string esql2 = "select value c from NorthwindEntities.Customers as c where c.Country='UK' order by c.CustomerID limit 10";
ObjectQuery<Customers> query2 = edm.CreateQuery<Customers>(esql2);
query1 = query1.Intersect(query2);
foreach (Customers c in query1)
{
Console.WriteLine(c.Country);
}
}

> Union/UnionAll:返回两个查询的合集,包括重复项。其中UnionAll必须是相同类型或者是可以相互转换的

> Include:可通过此方法查询出与相关的实体对象。实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql1 = "select value c from NorthwindEntities.Customers as c WHERE c.CustomerID ='HANAR'";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql1);
query1 = query1.Include("Orders");
foreach (Customers c in query1)
{
Console.WriteLine("{0},{1}", c.CustomerID, c.Orders.Count);
//输出:HANAR,14
} }

> OfType: 根据制定类筛选元素创建一个新的类型。此类型是要在实体模型中已定义过的。

> OrderBy

实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql1 = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql1);
query1.OrderBy("it.country asc,it.city asc");
//也可以这样写
//query1.OrderBy("it.country asc");
//query1.OrderBy("it.city asc");
foreach (Customers c in query1)
{
Console.WriteLine("{0},{1}", c.Country, c.City);
}
}

> Select

实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql1 = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql1);
ObjectQuery<DbDataRecord> records = query1.Select("it.customerid,it.country");
foreach (DbDataRecord c in records)
{
Console.WriteLine("{0},{1}", c[], c[]);
}
Console.WriteLine(records.ToTraceString());
//SQL输出:
//SELECT TOP (10)
//1 AS [C1],
//[Extent1].[CustomerID] AS [CustomerID],
//[Extent1].[Country] AS [Country]
//FROM [dbo].[Customers] AS [Extent1]
//ORDER BY [Extent1].[CustomerID] ASC
}

> SelectValue

实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql1 = "select value c from NorthwindEntities.Customers as c order by c.CustomerID limit 10";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql1);
ObjectQuery<string> records = query1.SelectValue<string>("it.customerid");
foreach (string c in records)
{
Console.WriteLine("{0}", c);
}
Console.WriteLine(records.ToTraceString());
//SQL输出:
//SELECT TOP (10)
//[Extent1].[CustomerID] AS [CustomerID]
//FROM [dbo].[Customers] AS [Extent1]
//ORDER BY [Extent1].[CustomerID] ASC
}

> Skip/Top

实例代码如下:

using (var edm = new NorthwindEntities())
{
string esql1 = "select value c from NorthwindEntities.Customers as c order by c.CustomerID ";
ObjectQuery<Customers> query1 = edm.CreateQuery<Customers>(esql1);
query1 = query1.Skip("it.customerid asc", "");
query1 = query1.Top("");
foreach (Customers c in query1)
{
Console.WriteLine("{0}", c.CustomerID);
}
Console.WriteLine(query1.ToTraceString());
//SQL输出:
//SELECT TOP (10)
//[Extent1].[CustomerID] AS [CustomerID]
//FROM [dbo].[Customers] AS [Extent1]
//ORDER BY [Extent1].[CustomerID] ASC
}

ObjectQuery查询及方法的更多相关文章

  1. EF ObjectQuery查询及方法

      string esql = "select value c from NorthwindEntities.Customers as c order by c.CustomerID lim ...

  2. Entity Framework 学习初级篇5--ObjectQuery查询及方法

    ObjectQuery 类支持对 实体数据模型 (EDM) 执行 LINQ to Entities 和 Entity SQL 查询.ObjectQuery 还实现了一组查询生成器方法,这些方法可用于按 ...

  3. SQO (标准查询运算符)方法 & Linq To Object

    #region SQO (标准查询运算符) 方法 #region Where() Find() FindAll() FirstOrDefault()等方法 static void c01where() ...

  4. Oracle 特殊字符模糊查询的方法

    最近在写DAO层的时候,遇到一个问题,就是使用like进行模糊查询时,输入下划线,无法精确查到数据,而是返回所有的数据. 这让我很好奇,百度之后才发现,原来是因为有些特殊字符需要进行转义才可以进行查询 ...

  5. SQL中对日期进行模糊查询的方法

    在我们通过SQL语句对数据库中的数据进行查询时,难免会遇到针对datetime的查询,但是因为一般情况下,输入的时间条件为年月日,所以,这种情况下,我们就要进行一下模糊查询,首先,摒弃一种投机取巧的方 ...

  6. MySQL、SQLServer2000(及SQLServer2005)和ORCALE三种数据库实现分页查询的方法

    在这里主要讲解一下MySQL.SQLServer2000(及SQLServer2005)和ORCALE三种数据库实现分页查询的方法. 可能会有人说这些网上都有,但我的主要目的是把这些知识通过我实际的应 ...

  7. SSH框架的多表查询(方法二)增删查改

     必须声明本文章==>http://www.cnblogs.com/zhu520/p/7773133.html  一:在前一个方法(http://www.cnblogs.com/zhu520/p ...

  8. eclipse F3可以查询某个方法的具体定义

    eclipse F3可以查询某个方法的具体定义

  9. ORACLE跨数据库查询的方法

    原文地址:http://blog.csdn.net/huzhenwei/article/details/2533869 本文简述了通过创建database link实现Oracle跨数据库查询的方法 ...

随机推荐

  1. json.net json转换神器

    json.nethttps://json.codeplex.com/ api documenthttp://james.newtonking.com/json/help/index.html#

  2. 整理收藏一份PHP高级工程师的笔试题

    整理了一份PHP高级工程师的笔试题,问题很全面.嗯,基本上这些题都答得不错,那么你应该可以胜任大部分互联网企业的PHP职位了.下面直接上题. 1. 基本知识点 HTTP协议中几个状态码的含义:503, ...

  3. poj 1273 Drainage Ditches【最大流入门】

    Drainage Ditches Time Limit: 1000MS   Memory Limit: 10000K Total Submissions: 63924   Accepted: 2467 ...

  4. 版本控制Subversion TortoiseSVN apache VisualSVN笔记(转载)

    转载于http://blog.sina.com.cn/s/blog_6b94d5680101m7ah.html Subversion(简称svn)是近年来崛起的版本管理软件,是cvs的接班人.目前,绝 ...

  5. 仰视源代码,实现memcpy

    C++实现内存的复制 通常我们使用深复制就是通过内存复制实现的,可是对象的复制涉及到基类派生类及其相关类的问题.这里不讨论. 目的为了可以明确内存复制的底层实现. void* memcpy(void* ...

  6. VMware虚拟机安装Linux英文改中文系统并更新yum安装输入法

    今天用VMware Workstation11时,出现了个"难题",在选择了系统盘后(我用iso镜像centos6.5)VMware会提示高速安装,安装后系统是英文版,并且找不到系 ...

  7. Linux系统及应用问题分析排查工具

    linux 阿里技术协会 摘要: Linux服务器上经常遇到一些系统和应用上的问题,如何分析排查,需要利器,下面总结列表了一些常用工具.trace tool:最后也列举了最近hadoop社区在开发发展 ...

  8. 【转】GitHub删除一个仓库——2013-08-27 21

    http://xiacijian.blog.163.com/blog/static/849931902012111195041170/ 1.进入要删除的仓库 2.找到 导航栏 Code  NetWor ...

  9. cglib源码分析(四):cglib 动态代理原理分析

    本文分下面三个部分来分析cglib动态代理的原理. cglib 动态代理示例 代理类分析 Fastclass 机制分析 一.cglib 动态代理示例 public class Target{ publ ...

  10. python--class test

    # !usr/bin/env  python3#-*- coding:utf-8 -*- 'a test class'class Student(object):    def __init__(se ...