ObjectQuery查詢及方法
ObjectQuery 類支持對 實體數據模型 (EDM) 執行 LINQ to Entities 和 Entity SQL 查詢。ObjectQuery 還實現了一組查詢生成器方法,這些方法可用於按順序構造等效於 Entity SQL 的查詢命令。下面是 ObjectQuery 的查詢生成器方法以及等效的 Entity SQL 語句:
Distinct,Except,GroupBy,Intersect,OfType,OrderBy,Select,SelectValue,Skip,Top,Union,UnionAll,Where
每個查詢生成器方法返回 ObjectQuery 的一個新實例。使用這些方法可以構造查詢,而查詢的結果集基於前面 ObjectQuery 實例序列的操作。下面來看具體的代碼片斷:
> 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); }
其中需要說明的是: MergeOption這個枚舉類型的參數項,MergeOption有四種值分別是:
> AppendOnly: 只追加新實體,不修改以前獲取的現有實體。這是默認行為。
> OverwriteChanges: 將 ObjectStateEntry 中的當前值替換為存儲區中的值。這將使用服務器上的數據重寫在本地所做的更改。
> PreserveChanges: 將替換原始值,而不修改當前值。這對於在發生開放式並發異常之後強制成功保存本地值非常有用。
> NoTracking: 將不修改 ObjectStateManager,不會獲取與其他對象相關聯的關系,可以改善性能。
> GetResultType方法:返回查詢結果的類型信息.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()); //輸出結果為: //NorthWindModel.Customers }
> ToTraceString方法:獲取當前執行的SQL語句。
> Where
實例代碼如下:
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的寫法 query1 = query1.Where("[email protected]"); query1.Parameters.Add(new ObjectParameter("customerid", "ALFKI")); //也可以這樣寫 //ObjectQuery<Customers> query2 = edm.Customers.Where("it.CustomerID=‘ALFKI‘"); foreach (var c in query1) Console.WriteLine(c.CustomerID); //顯示查詢執行的SQL語句 Console.WriteLine(query1.ToTraceString()); }
> First/ FirstOrDefault
實例代碼如下:
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
實例代碼如下:
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:返回兩個查詢的差集。實例代碼如下:
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); //輸出:UK } }
> Intersect:返回兩個查詢的交集。實例代碼如下:
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:返回兩個查詢的合集,包括重復項。其中UnionAll必須是相同類型或者是可以相互轉換的
> Include:可通過此方法查詢出與相關的實體對象。實例代碼如下:
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); //輸出:HANAR,14 } }
> OfType: 根據制定類篩選元素創建一個新的類型。此類型是要在實體模型中已定義過的。
> OrderBy
實例代碼如下:
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"); //也可以這樣寫 //query1.OrderBy("it.country asc"); //query1.OrderBy("it.city asc"); foreach (Customers c in query1) { Console.WriteLine("{0},{1}", c.Country, c.City); } }
> Select
實例代碼如下:
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輸出: //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
實例代碼如下:
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輸出: //SELECT TOP (10) //[Extent1].[CustomerID] AS [CustomerID] //FROM [dbo].[Customers] AS [Extent1] //ORDER BY [Extent1].[CustomerID] ASC }
> Skip/Top
實例代碼如下:
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輸出: //SELECT TOP (10) //[Extent1].[CustomerID] AS [CustomerID] //FROM [dbo].[Customers] AS [Extent1] //ORDER BY [Extent1].[CustomerID] ASC }
ObjectQuery查詢及方法