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

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

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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.

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Nội dung Text: Performing a SQL SELECT Statement and Storing the Rows Locally phần 2

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 Listing 5.1 shows a complete program that uses these steps. This program is named 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.

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