Tài liệu Performing a SQL SELECT Statement and Storing the Rows Locally phần 1 docx

Step 2: Create a SqlConnection Object to Connect to the Database Create a SqlConnection object to connect to the database, passing the connection string created in the previous step to

Performing a SQL SELECT Statement and Storing the Rows Locally

In the example featured in this section, you'll see how to connect to the SQL Server Northwind database and perform a SQL SELECT statement to retrieve the CustomerID, CompanyName, ContactName, and Address columns for the first 10 rows from the

Customers table These rows are stored in a DataSet object

Note Since I'll be using a SQL Server database, I'll use the SQL Server managed provider classes in the example

Outlining the Procedure

You can use the following steps to retrieve the rows into a DataSet object:

1 Formulate a string containing the details of the database connection

2 Create a SqlConnection object to connect to the database, passing the connection string to the constructor

3 Formulate a string containing a SELECT statement to retrieve the columns for the rows from the Customers table

4 Create a SqlCommand object to hold the SELECT statement

5 Set the CommandText property of the SqlCommand object to the SELECT string

6 Create a SqlDataAdapter object

7 Set the SelectCommand property of the SqlAdapter object to the SqlCommand object

8 Create a DataSet object to store the results of the SELECT statement

9 Open the database connection using the Open() method of the SqlConnection object

10 Call 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

11 Close the database connection, using the Close() method of the SqlConnection object created in step 1

12 Get the DataTable object from the DataSet object

13 Display the columns for each row in the DataTable, using a DataRow object to access each row in the DataTable

In the following sections, you'll learn the details of these steps and see example code

Step 1: Formulate a String Containing the Details of the Database Connection

When connecting to a SQL Server database, your string must specify the following:

• The name of the computer on which SQL Server is running You set this in the

server part of the string If SQL Server is running on your local computer, you can use localhost as the server name For example: server=localhost

• The name of the database You set this in the database part of the string For

example: database=Northwind

• The name of the user to connect to the database as You set this in the uid part of

the string For example: uid=sa

• The password for the database user You set this in the pwd part of the string For

example: pwd=sa

Note Typically, your organization's DBA (database administrator) will provide you with the appropriate values for the connection string The DBA is responsible for

administering the database

The following example creates a string named connectionString and sets it to an

appropriate string to connect to the Northwind database running on the local computer, using the sa user (with a password of sa) to connect to that database:

string connectionString =

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

Your connection string will differ based on how you connect to your Northwind database

Step 2: Create a SqlConnection Object to Connect to the Database

Create a SqlConnection object to connect to the database, passing the connection string created in the previous step to the constructor You use an object of the SqlConnection class to connect to a SQL Server database

The following example creates a SqlConnection object named mySqlConnection, passing connectionString (created in step 1) to the constructor:

SqlConnection mySqlConnection =

new SqlConnection(connectionString);

Step 3: Formulate a String Containing the SELECT Statement

Formulate a string containing the SELECT statement to retrieve the CustomerID,

CompanyName, ContactName, and Address columns for the first 10 rows from the

Customers table For example:

string selectString =

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

"FROM Customers "+

"ORDER BY CustomerID";

Note You use the TOP keyword in combination with an ORDER BY clause to retrieve the top N rows from a SELECT statement You can learn more about the TOP keyword

in Chapter 3

Step 4: Create a SqlCommand Object to Hold the SELECT Statement

You can call the CreateCommand() method of mySqlConnection to create a new

SqlCommand object for that connection The CreateCommand() method returns a new SqlCommand object for the SqlConnection object

In the following example, a new SqlCommand object named mySqlCommand is set to the SqlCommand object returned by calling the CreateCommand() method of

mySqlConnection:

SqlCommand mySqlCommand = mySqlConnection.CreateCommand();

Step 5: Set the CommandText Property of the SqlCommand Object to the SELECT

String

Set the CommandText property of your SqlCommand object to the SELECT string

created in step 4 The CommandText property contains the SQL statement you want to perform In the following example, the CommandText property of mySqlCommand is set

to selectString:

mySqlCommand.CommandText = selectString;

Step 6: Create a SqlDataAdapter Object

You use a SqlDataAdapter object to move information between your DataSet object and the database You'll see how to create a DataSet object in step 8 The following example creates a SqlDataAdapter object named mySqlDataAdapter:

SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter();

Step 7: Set the SelectCommand Property of the SqlAdapter Object to the

SqlCommand Object

The SelectCommand property contains the SELECT statement you want to run In the following example, the SelectCommand property of mySqlDataAdapter is set to

mySqlCommand:

mySqlDataAdapter.SelectCommand = mySqlCommand;

This enables you to perform the SELECT statement defined in mySqlCommand Step 10 actually performs the SELECT statement to retrieve rows from the database

Step 8: Create a DataSet Object to Store the Results of the SELECT Statement

You use a DataSet object to store a local copy of information retrieved from the database The following example creates a DataSet object named myDataSet:

DataSet myDataSet = new DataSet();

Step 9: Open the Database Connection Using the Open() Method of the

SQLConnection Object

The following example calls the Open() method for mySqlConnection:

mySqlConnection.Open();

Once you've opened the database connection, you can access the database

Step 10: Call the Fill() Method of the SqlDataAdapter Object to Retrieve the Rows

From the Table

Call the Fill() method of your SqlDataAdapter object to retrieve the rows from the

database, storing these rows locally in a DataTable of your DataSet object

The Fill() method is overloaded, and the version you'll see in the example accepts two parameters:

• A DataSet object

• A string containing the name of the DataTable object to create in the specified

DataSet