Tài liệu Performing a SQL SELECT Statement and Storing the Rows Locally phần 2 docx

The DataTable created in your DataSet is then populated with the rows retrieved by the SELECT statement.. The following example calls the Fill method of mySqlDataAdapter, passing myDataS

The Fill() method then creates a DataTable in the DataSet with the specified name and runs the SELECT statement The DataTable created in your DataSet is then populated with the rows retrieved by the SELECT statement

The following example calls the Fill() method of mySqlDataAdapter, passing myDataSet and "Customers" to the Fill() method:

mySqlDataAdapter.Fill(myDataSet, "Customers");

The Fill() method creates a DataTable object named Customers in myDataSet and

populates it with the rows retrieved by the SELECT statement You can access these rows, even when disconnected from the database

Step 11: Close the Database Connection

Close the database connection using the Close() method of the SqlConnection object created in the first step For example:

mySqlConnection.Close();

Note Of course, you don't have to immediately close the database connection before

reading locally stored rows from your DataSet I close the connection at this point

in the example to show that you can indeed read the locally stored rows- even when disconnected from the database

Step 12: Get the DataTable Object From the DataSet Object

Get the DataTable object created in step 10 from the DataSet object

You get a DataTable from your DataSet using the Tables property, which returns a

DataTableCollection object To get an individual DataTable from your DataSet, you pass the name of your DataTable in brackets ("Customers", for example) to the Tables

property The Tables property will then return your requested DataTable, which you can store in a new DataTable object that you declare In the following example,

myDataSet.Tables["Customers"] returns the Customers DataTable created in myDataSet

in step 10, and stores the returned DataTable in myDataTable:

DataTable myDataTable = myDataSet.Tables["Customers"];

Note You can also specify the DataTable you want to get by passing a numeric value to the Tables property For example, myDataSet.Tables[0] also returns the Customers DataTable

Step 13: Display the Columns for Each Row in the DataTable

Display the columns for each row in the DataTable, using a DataRow object to access each row in the DataTable The DataTable class defines a property named Rows that returns a DataRowCollection object containing the DataRow objects stored in that

DataTable You can use the Rows property in a foreach loop to iterate over the DataRow objects For example:

foreach (DataRow myDataRow in myDataTable.Rows)

{

// access the myDataRow object

}

Each DataRow object stores DataColumn objects that contain the values retrieved from the columns of the database table You can access these column values by passing the name of the column in brackets to the DataRow object For example,

myDataRow["CustomerID"] returns the value of the CustomerID column

In the following example, a foreach loop iterates over the DataRow objects in

myDataTable, and the column values are displayed for each row:

foreach (DataRow myDataRow in myDataTable.Rows)

{

Console.WriteLine("CustomerID = "+ myDataRow["CustomerID"]);

Console.WriteLine("CompanyName = "+ myDataRow["CompanyName"]);

Console.WriteLine("ContactName = "+ myDataRow["ContactName"]);

Console.WriteLine("Address = "+ myDataRow["Address"]);

}

As you can see, the name of each column is passed in brackets to each DataRow object, which then returns the column value

Note You can also specify the column you want to get by passing a numeric value in

brackets For example, myDataRow[0] also returns the CustomerID column value

Putting It All Together

SelectIntoDataSet.cs and is located in the ch05 directory

Listing 5.1: SELECTINTODATASET.CS

/*

SelectIntoDataSet.cs illustrates how to perform a SELECT statement

and store the returned rows in a DataSet object

*/

using System;

using System.Data;

using System.Data.SqlClient;

class SelectIntoDataSet

{

public static void Main()

{

// step 1: formulate a string containing the details of the

// database connection

string connectionString =

"server=localhost;database=Northwind;uid=sa;pwd=sa";

// step 2: create a SqlConnection object to connect to the

// database, passing the connection string to the constructor

SqlConnection mySqlConnection =

new SqlConnection(connectionString);

// step 3: formulate a SELECT statement to retrieve the

// CustomerID, CompanyName, ContactName, and Address

// columns for the first ten rows from the Customers table

string selectString =

"SELECT TOP 10 CustomerID, CompanyName, ContactName, Address "+ "FROM Customers " +

"ORDER BY CustomerID";

// step 4: create a SqlCommand object to hold the SELECT statement

SqlCommand mySqlCommand = mySqlConnection.CreateCommand();

// step 5: set the CommandText property of the SqlCommand object to

// the SELECT string

mySqlCommand.CommandText = selectString;

// step 6: create a SqlDataAdapter object

SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter();

// step 7: set the SelectCommand property of the SqlAdapter object

// to the SqlCommand object

mySqlDataAdapter.SelectCommand = mySqlCommand;

// step 8: create a DataSet object to store the results of

// the SELECT statement

DataSet myDataSet = new DataSet();

// step 9: open the database connection using the

// Open() method of the SqlConnection object

mySqlConnection.Open();

// step 10: use the Fill() method of the SqlDataAdapter object to

// retrieve the rows from the table, storing the rows locally

// in a DataTable of the DataSet object

Console.WriteLine("Retrieving rows from the Customers table");

mySqlDataAdapter.Fill(myDataSet, "Customers");

// step 11: close the database connection using the Close() method

// of the SqlConnection object created in Step 1

mySqlConnection.Close();

// step 12: get the DataTable object from the DataSet object

DataTable myDataTable = myDataSet.Tables["Customers"];

// step 13: display the columns for each row in the DataTable,

// using a DataRow object to access each row in the DataTable

foreach (DataRow myDataRow in myDataTable.Rows)

{

Console.WriteLine("CustomerID = "+ myDataRow["CustomerID"]);

Console.WriteLine("CompanyName = "+ myDataRow["CompanyName"]); Console.WriteLine("ContactName = "+ myDataRow["ContactName"]); Console.WriteLine("Address = "+ myDataRow["Address"]);

}

}

}

The output from this program is as follows:

Retrieving rows from the Customers table

CustomerID = ALFKI

CompanyName = Alfreds Futterkiste

ContactName = Maria Anders

Address = Obere Str 57

CustomerID = ANATR

CompanyName = Ana Trujillo Emparedados y helados

ContactName = Ana Trujillo

Address = Avda de la Constitución 2222

CustomerID = ANTON

CompanyName = Antonio Moreno Taquería ContactName = Antonio Moreno

Address = Mataderos 2312

CustomerID = AROUT

CompanyName = Around the Horn

ContactName = Thomas Hardy

Address = 120 Hanover Sq

CustomerID = BERGS

CompanyName = Berglunds snabbköp

ContactName = Christina Berglund

Address = Berguvsvägen 8

CustomerID = BLAUS

CompanyName = Blauer See Delikatessen ContactName = Hanna Moos

Address = Forsterstr 57

CustomerID = BLONP

CompanyName = Blondesddsl père et fils ContactName = Frédérique Citeaux

Address = 24, place Kléber

CustomerID = BOLID

CompanyName = Bólido Comidas preparadas ContactName = Martín Sommer

Address = C/ Araquil, 67

CustomerID = BONAP

CompanyName = Bon app'

ContactName = Laurence Lebihan

Address = 12, rue des Bouchers

CustomerID = BOTTM

CompanyName = Bottom-Dollar Markets ContactName = Elizabeth Lincoln

Address = 23 Tsawassen Blvd