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
}
本节，简单的介绍一下与ObjectQuery查询相关的语法，我个人觉得查询写法比较多，需要在日常的编程中去发现，在这里就不一一复述了。下节，将介绍EntityClient相关的内容。
