ObjectQuery查询及方法

Posted 2020-09-17 叶祖辉

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(), 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：

> 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("[email protected]");
    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[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
}

> 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", "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
}