Hướng dẫn học Microsoft SQL Server 2008 part 84 docx

Wellington FL All Things Techy 02/05/2007 M However, another way to return the first employee is to use theFirstproperty to manually select the first employeeelement: employees.Element

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LINQ to XML example

Create a new Visual Studio project On the form, add a text box and two buttons Set the properties of

the text box so that it is multi-line and has a vertical scroll bar Behind button1, add the following code:

XDocument riders = new XDocument (new XDeclaration("1.0", "utf-8", "yes"), new XComment("Riders for the year 2008"), new XElement("Riders",

new XElement("Rider", new XAttribute("Residence", "Florida"), new XElement("Name", "Ricky Carmich ael"), new XElement("Class", "450"),

new XElement("Brand", "Suzuki"), new XElement("Sponsors",

new XElement("Name", "Makita") )

), new XElement("Rider", new XAttribute("Residence", "California"), new XElement("Name", "Chad Reed"),

new XElement("Class", "450"), new XElement("Brand", "Yamaha"), new XElement("Sponsors",

new XElement("Name", "ProTaper") )

), new XElement("Rider", new XAttribute("Residence", "Mississippi"), new XElement("Name", "Kevin Windham"), new XElement("Class", "450"),

new XElement("Brand", "Honda"), new XElement("Sponsors",

new XElement("Name", "Factory Connection") )

), new XElement("Rider", new XAttribute("Residence", "Florida"), new XElement("Name", "James Stewart"), new XElement("Class", "450"),

new XElement("Brand", "Kawasaki"), new XElement("Sponsors",

new XElement("Name", "Renthal") )

) ) );

textBox1.Text = riders.ToString();

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Be sure to add the appropriate reference toSystem.XML.Linqin your form prior to running the

project:

using System.XML.Linq;

Before running the project, take a look at the code Notice that several of the classes were used to create

and define the XML For example, theXDocumentclass was used to create an XML document As well,

theXElementandXAttributeclasses were used to define the attributes and elements of the XML

document

Run the project and click button1 when the form displays The text box on the form will display the

following XML:

<! Riders for the year 2008 >

<Name>Ricky Carmichael</Name>

<Brand>Suzuki</Brand>

<Name>Makita</Name>

</Sponsors>

</Rider>

<Brand>Yamaha</Brand>

<Name>ProTaper</Name>

</Sponsors>

</Rider>

<Name>Kevin Windham</Name>

<Brand>Honda</Brand>

<Name>Factory Connection</Name>

</Sponsors>

</Rider>

<Name>James Stewart</Name>

<Brand>Kawasaki</Brand>

<Name>Renthal</Name>

</Sponsors>

</Rider>

</Riders>

Isn’t that much easier than using the DOM?

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What about querying XML? Open Notepad or another text editor and type in the following:

<Name>Scott</Name>

<City>Wellington</City>

</Address>

<Title>All Things Techy</Title>

</Employee>

<Name>Steve</Name>

<City>Snahomish</City>

</Address>

<Title>Mr SciFi</Title>

</Employee>

<City>Easley</City>

</Address>

<Title>All Things Bleeding Edge</Title>

</Employee>

</Employees>

Save this file asEmployees.xml Next, add the following code behind button2 :

XElement employees = XElement.Load("Employees.xml");

textBox1.Text = employees.Element("Employee").ToString();

Run the project again and click button2 The text box will be filled with the first employee from the

XML document, as shown here:

<Name>Scott</Name>

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<City>Wellington</City>

</Address>

<Title>All Things Techy</Title>

</Employee>

However, another way to return the first employee is to use theFirst()property to manually select

the first employeeelement:

employees.Elements("Employee").First()

Another alternative is to use theElementAt()method to specify which element to return:

employees.Elements("Employee").ElementAt(0)

Yet another method is to use a LINQ query expression:

XElement empnum2 = (from emp in employees.Elements("Employee")

where (int)emp.Attribute("id") == 2 select emp).ElementAt(0);

This example uses a LINQ query expression to query the XML document, using the attribute"id"as

a filter In this example, theElementAt()method is used to return the first element that matches the

specified criteria

This next example uses a LINQ query expression to query the XML document and return the values of

all theNameelements for each employee To get that information, thedescendants()method is used

to return a collection of all descendants for the selected element:

IEnumerable<string> empNames =

from emp in employees.Descendants("Name")

orderby emp.Value

select emp.Value;

foreach (string name in empNames)

listBox1.Items.Add(name);

When run, this example returns the following:

Joe

Scott

Steve

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Traversing XML

Traversing XML in an XML tree using LINQ to XML is quite simple Just use the methods of the

XElementandXAttributeclasses as necessary Basically, theElementsandElementmethods

provide all of the element children of anXContainer(anXElementorXDocument) object Using

theXNameobject, such asElement(XName), one can return elements of that specificXName

For example, using the XML document used in the previous example, one can ‘‘walk the XML tree,’’ as

shown here:

employees.Element("Employees").Element("Employee") employees.Element("Employees").Element("Employee") Element("Name")

The same can be accomplished with attributes as well The following illustrates how to walk an XML

tree to get to a specific attribute:

employees.Elements("Employee").ElementAt(1).Attribute("id")

The thing to remember is that theNodes(),Elements(),Element(Name), andElements(Name)

methods provide the foundation and basic functionality of XML tree navigation

LINQ to DataSet

Most, if not all, NET developers are familiar with the concept of aDataSetbecause it is one of the

most frequently used components in ADO.NET ADataSetis a representation of the tables and

rela-tionships found in a database, exposing a hierarchical object model made of objects such as tables, rows,

columns, constraints, and relationships

TheDatasetitself is extremely flexible and powerful It provides the capability for applications to

efficiently work with a subset of data found in a database and to manipulate the data as needed by the

application, all while in a disconnected state

Yet, with all the flexibility there has not been, up to this point, a means or method for querying data

contained within theDataSet(other than the few methods on theDataTableclass such asSelect,

GetParentRow, andGetChildRow) This is where LINQ and LINQ toDataSetcome in With the

querying power of LINQ, LINQ toDataSetprovides developers with a full set of query capabilities to

quickly and easily query the contents of aDataSet

An illustration of how easy it is to query aDataSetusing LINQ to DataSet follows

Querying a DataSet using LINQ to DataSet

First, create a new Visual Studio project Make sure that the project includes references to both the

System.CoreandSystem.Data.DataSetExtensionnamespaces On the form, place a button, a

text box, and a list box Add the followingusingstatement to the code as well:

using System.Data.SqlClient;

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Anyone who has worked with aDataSetis intimately familiar with how to populate it using the

SqlDataAdapter The following example uses theSqlDataAdapterto populate aDataSetwith

data from thePerson.Contacttable then use a LINQ query expression to query theDataSet

Add the following code to the Click event of the button:

try

{

//first, populate the dataset

DataSet ds = new DataSet();

string connectionInfo = "Data Source=SERVERNAME;Initial

Catalog=AdventureWorks;Integrated Security=sspi";

SqlDataAdapter da = new SqlDataAdapter("select pc.ContactID,

pc.FirstName, pc.MiddleName, pc.LastName, pc.EmailAddress from

Person.Contact pc", connectionInfo);

da.TableMappings.Add("Table", "Contact");

da.Fill(ds);

DataTable dt = ds.Tables["Contact"];

textBox1.Text = ds.Tables[0].Rows.Count.ToString();

//now query it for specific people

var con = from contact in dt.AsEnumerable()

where contact.Field<string>("FirstName") StartsWith("S")

select new { ContactID = contact.Field<int>

("ContactID"),

FirstName = contact.Field<string>

("FirstName"), MiddleName = contact.Field<string>("MiddleName"),

LastName = contact.Field<string>

("LastName"), Email = contact.Field<string>

("EmailAddress")};

foreach (var cont in con)

listBox1.Items.Add(cont.ContactID.ToString() + " " + cont.FirstName + " " +

cont.MiddleName + " " + cont.LastName + " " + cont.Email);

}

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catch (Exception ex) {

MessageBox.Show(ex.Message);

}

Compile the project to ensure there are no errors, and then run it When the form opens, click the

but-ton The text box will display the total number of records contained in theDataSet The list box will

fill with all the contacts from theDataSetwhose first name begins with the letter S

Note a couple of things here:

■ TheDataSetis not a typedDataSetbecause theField()method is used to access the column values of theDataRow

■ The LINQ query itself Like the others before it in this chapter, the syntax follows the same pattern as the other queries The only difference is the data source

■ The use of theAsEnumerable()method This method returns anIEnumerableobject, which in this case enables you to iterate over the data in theDataTable

The next example does two things One, it uses a typedDataSet Two, it adds a second table to the

DataSetthat illustrates how to use a LINQ query to query data from two tables

Add a new item to the solution, this time adding aDataSet In the Add New Item dialog, select Data

from the list of Categories, and then selectDataSetfrom the list of Templates Name theDataSet

ContactDS.xsdand click OK

The DataSet Designer will display in the Visual Studio IDE The DataSet Designer is a visual tool for

creating and editing typedDataSets and the individual items that make upDataSets The DataSet

Designer provides developers with a visual depiction of the objects contained in theDataSets

In the Dataset Designer, drag and drop theContactandEmployeetables from the Server Explorer

Next, modify the code behind the button as follows:

try { ContactsDS ds = new ContactsDS();

string connectionInfo = "Data Source=SERVERNAME;Initial Catalog=AdventureWorks;Integrated Security=sspi";

SqlDataAdapter da = new SqlDataAdapter("select * from Person Contact; select * from HumanResources.Employee;", connectionInfo);

da.TableMappings.Add("Table1", "Employee");

da.Fill(ds);

textBox1.Text = "contact table has " + ds.Tables[0].Rows.Count ToString() + " records, and employee table has " +

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ds.Tables[1].Rows.Count.ToString() + " records.";

var query =

from con in ds.Contact

join emp in ds.Employee

on con.ContactID equals emp.ContactID

where con.FirstName.StartsWith("S")

select new

{

ContactID = con.ContactID, FirstName = con.FirstName, LastName = con.LastName, EMail = con.EmailAddress, EmpID = emp.EmployeeID, IDNum = emp.NationalIDNumber, HireDate = emp.HireDate };

foreach (var cont in query)

listBox1.Items.Add(cont.ContactID.ToString() + " " +

cont.FirstName + " " + cont.LastName + " " + cont.EMail + " " + cont.EmpID + " " + cont.IDNum + " " + cont.HireDate.ToString());

}

catch (Exception ex)

{

}

Compile the project to ensure there are no coding errors, and then run it When the form displays, click

the button The text box will display a message indicating the number of records returned in each table

The list box will then display a combination of contact and employee data for all contacts whose first

name begins with the letter S

Before moving on to the last example, consider what the code is doing In many ways it is very similar

to the previous example, but several things are different:

■ This example uses a typedDataSet

■ Two tables are being populated, not just one

■ Because a typedDataSetis being used, it’s no longer necessary to use theField<>method

Instead, it employs a direct mapping to the tables and columns and the use of IntelliSense

■ The LINQ query expression uses thejoinstandard query operator to join theContacttable

and theEmployeetable

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One more comment about the prior example Because a typedDataSetwas used, theDataSetitself

contains the relationship information This can be visually seen on the DataSet Designer If a typed

datasethad not been used, then the relationship would have to be defined programmatically using

theDataRelationclass

Data binding with LINQ to DataSet

This section describes how to use LINQ to DataSet to do data binding This example is similar to the

first, in that it uses an untypedDataSet, but this is not the focus of the example because a typed

DataSetcan still be used The important thing to learn from this example is how to do data binding

using LINQ to DataSet

For this example, open the form in design view From the toolbox, drag and drop aDataGridViewcontrol

onto the form Place another button on the form and enter the following code in its Click event:

try { DataSet ds = new DataSet();

string connectionInfo = "Data Source=SERVERNAME;Initial Catalog=AdventureWorks;Integrated Security=sspi";

SqlDataAdapter da = new SqlDataAdapter("select pc.ContactID, pc.FirstName, pc.MiddleName, pc.LastName, pc.EmailAddress from Person.Contact pc", connectionInfo);

da.Fill(ds);

DataTable dt = ds.Tables["Contact"];

textBox1.Text = ds.Tables[0].Rows.Count.ToString();

IEnumerable<DataRow> contact = from con in dt.AsEnumerable() where con.Field<string>("FirstName").StartsWith("S") orderby con.Field<string>("LastName")

select con;

DataTable partialTable = contact.CopyToDataTable<DataRow>();

DataView dv = new DataView(partialTable);

dataGridView1.DataSource = dv;

} catch (Exception ex) {

}

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This example populates aDataSetwith all the data from theContactstable and then queries the

DataSetusing a LINQ query The query returns an enumeration ofDataRowobjects that is used to

populate aDataTablevia theCopyToDataTable()method Once theDataTableis populated,

a newDataViewis created and populated with the data in theDataTable TheDataViewis then

assigned to the DataSourceproperty of theDataGridView

Another option for binding would have been to bind theDataTabledirectly to theDataGridView,

as follows:

dataGridView1.DataSource = partialTable;

However, theDataViewprovides additional capabilities such as sorting and filtering data that is stored

in aDataTable

Additionally, the data can be bound by implicitly binding data to controls, by implementing the

IListSourceinterface This interface enables an object to return a list that is bindable to a data

source, as shown here:

var query =

from c in contact

where c.Field<string>("FirstName").StartsWith("S")

orderby c.Field<string>("LastName")

select c;

dataGridView1.DataSource = query;

Likewise, the following can be done:

BindingSource bindsrc = new BindingSource();

bindsrc.DataSrouce = query;

dataGridView1.DataSource = bindsrc;

Implicit binding is available because theTable<T>andDataQuery<T>classes have been updated to

implement theIListSourceinterface

All of these examples populated the initial DataSet with all of the records from the

Contacts table This was done by design to illustrate the querying capabilities of LINQ to

DataSet.

LINQ to Entities

When talking about LINQ to Entities, the discussion must also discuss the ADO.NET Entity Framework

This is because LINQ to Entities is the part of the ADO.NET Entity Framework that provides the LINQ

query capabilities

That statement then begs the question, what is the difference between LINQ to SQL and LINQ to

Enti-ties? In the LINQ to SQL section, it was stated that LINQ to SQL is an object-relational mapping

frame-work that enables the direct 1-to-1 mapping of SQL Server database objects to NET classes, where the

database closely resembles the application model

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