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

http://www.pasou.cn/edu/html/List8091_1.html（更多内容请查看） 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：l　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(), 10);

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：l　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。l　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。l　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。l　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。l　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

}

l　ToTraceString方法：获取当前执行的SQL语句。l　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());

}

l　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);

}

l　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);}}

l　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}}

l　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);}}

l　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。

l　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}}

l　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。

l　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);}}

l　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[0], c[1]);}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}

l　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}

l　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", "10");query1 = query1.Top("10");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}