Hướng dẫn học Microsoft SQL Server 2008 part 83 pptx

Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 782 Part V Data Connectivity (4) WHERE (5) GROUP BY (6) WITH (7) HAVING (10) ORDER BY Notice that the FROM clause is processed first, while the SELECT clause is processed almost last. Any clause that is not specified in the query is simply skipped by the query processing engine. This informa- tion is important because it points out the similarities between LINQ query syntax and how SQL Server processes a query. For more information on SQL logical query flow, please see Chapter 8, ‘‘Introducing Basic Query Flow.’’ In a LINQ query, the first clause is the FROM clause, which specifies the source of the data, and it must end with either a SELECT clause or a GROUP clause. The first clause, called a generator, defines where the data will be coming from when the query is executed. It also specifies the range variable that is used as a reference for each element in the data source. For example, the following uses LINQ to return a list of the directories in the root of a hard drive: DirectoryInfo di = new DirectoryINfo("C:\\"); var query = from dir in di.GetDirectories() order by di.Name select new { dir.Name }; foreach (var item in query) Console.WriteLine(item.Name); In this example, the first statement creates a new instance of the DirectoryInfo class. This is what is used to retrieve a list of the directories. The second statement is the LINQ statement. The variable var is the query variable. This variable does not store actual result data. The results are produced during the iteration of the query, which is done in the foreach loop. When the foreach statement executes, the query results are returned during each iteration in the variable item. Whereas the LINQ query and the T-SQL query are executed similarly, the T-SQL query syntax is a bit different. In T-SQL, the query would be executed internally following the steps described earlier. With LINQ, the query does not need to go through the rewriting process. In the next example, the LINQ query uses the same syntax to query data from the Contact table in the AdventureWorks database in SQL Server: var query = from c in Contact where c.FirstName.StartsWith("S") orderby c.LastName select c; 782 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 783 LINQ 34 Notice in this query that the syntax is exactly the same as the first query. An additional WHERE clause has been added, but syntactically it is the same. This is what is great about LINQ. Similar query expres- sions can be written against two completely different sources of data. Query syntax and method syntax LINQ has the ability to write queries using both query syntax and method syntax. The examples shown so far have used query syntax, which is writingthequeryasaqueryexpression,asfollows: IEnumberable<string> query = from c in Contact where c.FirstName.StartsWith("S") select c; This declarative syntax is easy to read and understand, but developers also have the option of writing queries using method syntax. When a LINQ query is compiled, the query expression is translated into method syntax because the .NET CLR (common language runtime) doesn’t understand query syntax. Thus, at compile time, query expressions are translated into method calls because this is what the CLR understands. The following shows the previous query written using method syntax: IEnumberable<string> query = contact.Where(c => c.FirstName.StartsWith("S")); For clarification, take a look at a couple more examples. This first example adds an additional filter to the previous example: IEnumberable<string> query = from c in Contact where c.FirstName.StartsWith("S") && c.LastName.StartsWith("A") select c; Hereisthesamequeryinmethodsyntax: IEnumberable<string> query = contact.Where(a => a.FirstName.StartsWith("S") && a.LastName.StartsWith("A")); Just to complicate things a bit more, this next example adds an OrderBy clause: IEnumberable<string> query = from c in Contact where c.FirstName.StartsWith("S") && c.LastName.StartsWith("A") order by c.LastName select c; 783 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 784 Part V Data Connectivity The preceding query written using method syntax is as follows: IEnumberable<string> query = contact.Where(c => c.FirstName.StartsWith("S") && c.LastName.StartsWith("A")) .OrderBy(c => c.LastName); The output of both queries is identical. Choosing a query syntax As you have seen, although there are syntactical differences between query syntax and method syntax, execution is essentially the same. However, the .NET CLR does not understand query syntax, so it is translated into method syntax anyway. The CLR understands method calls, so query expressions are translated into method syntax (method calls) and executed as such. Although there is no semantic difference between method syntax and query syntax, I recommended that developers use query syntax whenever possible, as it is easier to read, understand, and maintain. Now that you have a good understanding of LINQ, the standard query operators, and LINQ query expressions, the rest of this chapter focuses on the LINQ providers and offers detailed examples of each, beginning with LINQ to SQL. LINQ to SQL LINQ to SQL is part of ADO.NET which provides a run-time infrastructure for mapping relational data as objects. In essence, it is an ORM (object-relational mapping) framework that enables the direct 1-to-1 mapping of a SQL Server database and its objects to .NET classes without losing the ability to query. One of most important things that developers need to understand when working with LINQ to SQL is the DataContext class. The DataContext is the main medium through which objects are retrieved from the database and changes are resubmitted. The DataContext is much like an ADO.NET connec- tion in that it is initialized with a connection string. The DataContext has the responsibility of converting objects, which means that because LINQ to SQL deals with objects, the DataContext converts the objects into SQL queries, as well as reassembling the results back into queryable objects. The DataContext has several overloads, one of which is just a simple connection string specifying the connection information, as shown here: DataContext db = new DataContext("Initial Catalog=AdventureWorks; i Integrated Security=sspi"); It is recommended, however, that you use strongly typed DataContexts. This is as simple as creating a new class that inherits from the DataContext class: [Database(Name = "AdventureWorks")] public class AdventureWorks : DataContext { public AdventureWorks(string connection) : base(connection) {} } 784 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 785 LINQ 34 Here, the strongly typed DataContext is given the same name as the database to use — in this case, the AdventureWorks (2005 and 2008) database. Therefore, no database needs to be specified in the connection string. In the preceding example, the [Database] attribute is applied to any strongly typed DataContext declaration. This attribute is optional, but when it is used a Name property must also be used to sup- ply the name of the database to which you are mapping. This attribute maps the database in the Name property to your entity class. With the DataContext created, the next step is to create mappings to the specific objects. Database tables are represented by entity classes in LINQ to SQL. An entity class is a normal class annotated with a specific tag that maps that class to a specific table in the database. The Table attribute, part of the System.Data.Linq.Mapping namespace, maps an entity class to a database table or view. This attributehasasingleproperty, Name, which specifies the name of the rela- tional table or view. In the following example, the Table attribute is applied to a class to define a mapping between the Person.Contact table in the AdventureWorks database and a class named Contact: [Table(Name="Person.Contact")] public class Contact { // } Once the table is mapped to an entity class, table columns must be mapped to class properties. This is accomplished by applying the Column attribute, which maps a database table column to a member of an entity class, as shown here: [Column(DbType="nvarchar(50) not null")] public string FirstName Several properties are commonly applied with this attribute, including the following: ■ Name: The name of the table column ■ DbType: Database type of the column ■ IsPrimaryKey: Specifies that the associated column is the primary key of the corresponding table ■ IsDbGenerated: Specifies that the associated column auto-generates its values ■ CanBeNull: Specifies that the associated column can contain null values So far, the connection has been defined via the DataContext, and the table and column mappings have been defined. When this is all put together, it looks like the following: public class AdventureWorks : DataContext { public AdventureWorks(string connection) : base(connection) {} public Table<Contact> Contact; } 785 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 786 Part V Data Connectivity [Table(Name="Person.Contact")] public class Contact { [Column(DbType="nvarchar(50) not null")] public string FirstName } To help this sink in, this section offers a couple of examples. The first example applies the mappings manually, and the second example illustrates an easier way. Example 1: Manually applying the mappings For this first example, follow these steps: 1. Create a new Visual Studio C# Windows project. 2. Add a reference to the System.Data.Linq component. 3. On the form, add a button and a list box. 4. Add a new class to the project and name it AdventureWorks. 5. Open the new class and replace all the code that is there with the following code. (Because I named my project LINQ, the namespace is called LINQ. Be sure to change the namespace name if you named your project something different.) using System.Data.Linq; using System.Data.Linq.Mapping; using System.Data; using System.Collections.Generic; using System.Reflection; using System.Linq; using System.Linq.Expressions; using System.ComponentModel; using System; namespace LINQ { [Database(Name = "AdventureWorks")] public class AdventureWorks : DataContext { public AdventureWorks(string connection) : base(connection) { } public Table<Contacts> Contact; } [Table(Name="Person.Contact")] public class Contacts { [Column(DbType = "int not null", IsDbGenerated = true, IsPrimaryKey = true)] 786 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 787 LINQ 34 public int ContactID; [Column(DbType = "nvarchar(8)")] public string Title; [Column(DbType = "nvarchar(50)")] public string FirstName; [Column(DbType = "nvarchar(50)")] public string MiddleName; [Column(DbType = "nvarchar(50)")] public string LastName; [Column(DbType = "nvarchar(50)")] public string EmailAddress; [Column(DbType = "int")] public int EmailPromotion; [Column(DbType = "bit")] public byte NameStyle; } } This code first defines the DataContext class. The next section of code defines a class that maps that class to the Person.Contact table (in SQL Server 2005) as well as maps several columns from the table to members of the class. Note some of the properties used in the Column attribute applied to the ContactID mem- ber. The ContactID column in the Person.Contact table is a primary key column that auto-generates its IDs. Thus, in order for LINQ to support this and recognize this functionality, the IsDbGenerated and IsPrimaryKey properties need to be used. 6. In the Click event of the button on the form, add the following code: AdventureWorks db = new AdventureWorks("Data Source=ServerName; Initial Catalog=AdventureWorks;Integrated Security=sspi"); var query = from con in db.Contact where con.FirstName.StartsWith("S") select con; foreach (var cont in query) listBox1.Items.Add(cont.FirstName); 7. Compile the project to ensure that there are no errors, and then run it. When the form dis- plays, press the button. The list box should be populated with the first name of all contacts whose first name begins with the letter S. Prior to moving on to the next example, let’s take a closer look at this one. The class created earlier used LINQ to SQL to define the mapping between the relational data and the class. On the form, an 787 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 788 Part V Data Connectivity instance of the class was created, and a LINQ query was created and used, employing a few of the standard query operators. The query was then iterated over, at which point the data was returned to the application. The thing to pay attention to here is the query itself. As the query was typed, IntelliSense displayed the column names of the table, as well as a list of the available standard query operators and methods. This is LINQ in all its glory! Equally, notice that the syntax used to retrieve relational data is exactly the same as what was used to query the directories on the local drive in the example shown earlier. This first example demonstrated how to create the mapping classes manually in order to provide a good understanding of how the mapping works. However, there is an easier way. Example 2: The easy way For this second example, follow these steps: 1. In Visual Studio, right-click the project in the Solution Explorer and select Add ➪ New Item. 2. When the Add New Item dialog appears, select the LINQ to SQL Classes item from the list of templates. Name it AWDB.dbml and click OK. A LINQ to SQL designer page will open in the Visual Studio environment. This object- relational designer provides a visual surface for creating LINQ to SQL entity classes and rela- tionships based on database objects. In essence, this is where the object model is defined that maps database objects within an application. 3. The empty design surface represents a DataContext that is ready to be configured. From here, open the Server Explorer and, if necessary, create a connection to the AdventureWorks database. Once that is created, database objects can be dragged onto the designer. 4. For this example, drag the Person.Contact table onto the designer and then save the project. Next, modify the code behind the button on the form to look like the following: AWDBDataContext db = new AWDBDataContext(); var query = from con in db.Contacts where con.FirstName.StartsWith("S") select con; foreach (var cont in query) listBox1.Items.Add(cont.FirstName); The only thing that changes is the first line; everything else remains the same. Run the project and click the button. The list box will be populated with the same data presented in the first example. 5. Before moving on, however, a few things need to be highlighted. In the Solution Explorer, expand the AWDB node. Underneath that node are two files: AWDB.dbml.layout and AWDB.designer.cs. The layout file simply contains the layout information for the designer. If you open this file you will see that it is simply an XML file. The important file is the designer.cs file. Open this file and notice that it looks very similar to the file manually created in the first example. It contains a bit more information than the one you manually created earlier, but overall it is the same. 788 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 789 LINQ 34 One of the differences between the two examples is that connection string information needed to be passed in the first example, whereas no connection information needed to be passed in the second example. This is because the Server Explorer connection information was automatically saved and added to the project’s resource file when the OR mapping was created. In the designer.cs file, the DataContext class contains the following: public AWDBDataContext() : base(global::LINQ.Properties.Settings.Default .AdventureWorksConnectionString, mappingSource) { OnCreated(); } Right-click on the line AdventureWorksConnectionString and select Go To Definition, which will take you to where the connection string is defined in the project settings. Notice that the AWDBDataContext has several overloads, which enables different connection information to be passed to the DataContext. Much more could be said about LINQ to SQL, but this should provide a solid foundation for under- standing how it works. Your homework assignment for this section is to add more tables to the designer, look at their relationships, and modify the LINQ query to utilize the relationships (hint: use the JOIN operator). LINQ to SQL and Stored Procedures A few words on using LINQ to SQL and stored procedures. I am a firm believer in using stored procedures for several reasons: ■ They provide a layer of abstraction. ■ They provide increased maintainability. ■ They enable performance improvement through database query optimizations. The great thing about LINQ is that it supports accessing data by calling and executing stored procedures. For example, given the following stored procedure: CREATE PROCEDURE [dbo].[uspGetSalesOrderDetail] @SalesOrderID [int] AS BEGIN SET NOCOUNT ON; SELECT SalesOrderDetailID, OrderQty, continued 789 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 790 Part V Data Connectivity continued ProductID, UnitPrice, LineTotal FROM Sales.SalesOrderDetail WHERE SalesOrderID = @SalesOrderID END; Calling a stored procedure is as simple as the following: var query = db.uspGetSalesOrderDetail(43659) foreach (SalesOrderDetailResult salesOrderDetail in query) { //do something } However, while using stored procedures does have its advantages, it is still an individual call as to when and where stored procedures are appropriate. For example, if I know that a query will always return a single row (and maybe only few columns), then I may forgo using a stored procedure in that case. If I have a complex T-SQL query, then I will more than likely use a stored procedure. My recommendation is to experiment and find what works best given the scenario in your application. Using stored procedures provides certain benefits, such as those listed earlier, but not all scenarios need stored procedures; and as you become familiar with LINQ and LINQ to SQL, you will find what works best in each situation. LINQ to XML I love XML. The fact that it is entirely possible to query XML using LINQ just rocks my world. It is an excellent form of moving and storing data. Those who have used other technologies to query XML know that it can be somewhat difficult and cumbersome. LINQ to XML is heaven sent. This section introduces LINQ to XML and demonstrates how easy it is to create and query XML using LINQ to XML. The component that gives LINQ to XML its power is the System.Xml.Linq namespace, and its corresponding classes. These classes provide the capability to work with XML with ease, leav- ing behind the need to work with complex and cumbersome technologies such as the DOM and XQuery. Table 34-2 describes the 19 classes contained in the System.Xml.Linq namespace that enable devel- opers to easily work with XML. 790 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 791 LINQ 34 TABLE 34-2 LINQ to XML Classes Class Description XAttribute Represents an XML attribute XCData Represents a CDATA text node XComment Represents an XML comment XContainer An abstract base class representing nodes that have child nodes XDeclaration Represents an XML declaration XDocument Represents an XML document XDocumentType Represents an XML DTD XElement Represents an XML element XName Represents the name of an XML element or attribute Xnamespace Represents an XML namespace XNode An abstract class representing nodes of an XML element tree XNodeDocumentOrderComparer Provides mechanisms for node comparisons regarding their order within the XML document XNodeEqualityComparer Provides mechanisms for node comparisons regarding their equality value XObject An abstract class representing XNodes and XAttributes XObjectChange The event type when an XObject event is raised XObjectChangeEventArgs Provides information and data for the Changing and Changed events XObjectChangeEventHandler The method that handles the XObject’s Changed and Changing events XProcessingInstruction Represents an XML processing instruction XText Represents an XML text node 791 www.getcoolebook.com . the LINQ providers and offers detailed examples of each, beginning with LINQ to SQL. LINQ to SQL LINQ to SQL is part of ADO.NET which provides a run-time infrastructure for mapping relational. in the variable item. Whereas the LINQ query and the T -SQL query are executed similarly, the T -SQL query syntax is a bit different. In T -SQL, the query would be executed internally following the. in SQL Server: var query = from c in Contact where c.FirstName.StartsWith("S") orderby c.LastName select c; 782 www.getcoolebook.com Nielsen c34.tex V4 - 07/23/2009 1:56pm Page 783 LINQ