Shulou(Shulou.com)06/02 Report--

This article mainly introduces what is the use of DataAdapter class in VB.NET database. It is very detailed and has certain reference value. Friends who are interested must finish it.

DataAdapter class

DataAdapter is a subcomponent of the data provider. Like the Connection and Command classes, the DataAdapter class exists in ODBC, OLE DB, SQL Server, and Oracle. This section introduces the DataAdapter class of OLE DB, and note that different versions of the DataAdapter class have the same functionality.

The DataAdapter class is the bridge between the database and the program. It can execute Command objects, retrieve data from the database, populate it into DataSet objects, or use DataSet objects to insert, update, and delete data in the database.

DataAdapter is used to populate DataSet objects, so this section begins to explore DataSet objects in detail. Subsequent chapters will further introduce the DataSet object and discuss more knowledge and usage of this class.

DataAdapter is most commonly used to retrieve data from a database and populate DataSet objects, so first look at the constructors for this purpose.

The * constructors of the Command class use SQL strings and Connection objects. The following code uses the Command object in the previous example in the constructor of DataAdapter.

The next line of code declares a new object that represents the DataSet class, and notice that DataSet is independent of the provider because it does not have the prefix ODBC, OLE DB, SQL, or Oracle. The constructor of DataSet provides an overloaded list, but typically initializes without arguments.

After initializing the DataAdapter and DataSet objects, you retrieve the data from the database and populate the DataSet object. Use the Fill method of DataAdapter to complete the padding. The Fill method also provides an overloaded list, but the most common is the Fill method in the following code. This method specifies the object that represents the DataSet and a table name that is used for table mapping when multiple tables are to be added to the DataSet object. This table name can also be used to reference a table in a DataSet object. Note that you do not have to use the specified table name in the FROM clause of the SQL statement, you can use any table name.

After populating the DataSet object with data, the work of DataAdapter is done, and the Dispose method should be called to release the resources occupied by DataAdapter and set it to Nothing. Command's work is done, and * deletes it and sets it to Nothing to release resources. In addition, if no more database operations are performed, close the database connection and call the Dispose method on the connection.

Dim objDataAdapter As New OleDbDataAdapter (objCommand) Dim objDataSet As New DataSet objDataAdapter.Fill (objDataSet, "Employees") objDataAdapter.Dispose () objDataAdapter = Nothing objCommand.Dispose () objCommand = Nothing

Another common constructor for DataAdapter passes the SQL statement directly to DataAdapter instead of using the Command object, as shown in the following code. In this constructor, you pass a string variable and an object that represents a database connection. In the previous example, the Command object is passed to DataAdapter so that DataAdapter can extract the connection information from it. In this constructor, a string is used for the SQL statement, so DataAdapter needs to know how to communicate with the database through the Connection object to execute the SQL string.

Dim strSQL As String = _ "SELECT FIRST_NAME, LAST_NAME, FROM HR.EMPLOYEES" Dim objDataAdapter As New OleDbDataAdapter (strSQL, objConnection) Dim objDataSet As New DataSet objDataAdapter.Fill (objDataSet, "Employees") objDataAdapter.Dispose () objDataAdapter = Nothing

After you populate the DataSet object with data, you can process the data. The DataSet object contains a series of tables, each table contains a series of rows, and each row contains a series of items that represent the columns in the row. This sounds confusing, but it's actually very simple, as shown in the following code.

The code declares an object for DataRow that accesses each row in the Rows collection, which is placed in the table of the Tables collection. Then use the For Each loop to iterate over the rows in the representation. Using the Item property of the DataRow object, you can access each column in the row, as shown in the following code. This line of code outputs the name of each employee in the DataSet object to the IDE output window.

* after using the DataSet object, call the Dispose method to release the resources occupied by the DataSet object and set it to Nothing. This is important because the DataSet object represents an in-memory data cache, that is, all the data contained in the DataSet object is loaded into memory, so the memory should be freed as soon as possible.

Dim objDataRow As DataRow For Each objDataRow In objDataSet.Table ("Employees") .Rows Debug.WriteLine (objDataRow.Item ("FIRST_NAME") & "& _ objDataRow.Item (" LAST_NAME ") Next objDataSet.Dispose () objDataSet = Nothing

After you modify the data in the DataSet object, you can use the Update method of DataAdapter to update the data in the DataSet object to the database. This method is rarely used because views or stored procedures are typically used to populate DataSet with data from multiple tables. When you update or insert data in a relational database, you typically use stored procedures to perform insert or update operations, and then use Command objects to execute these stored procedures. Stored procedures can execute one or more SQL statements and perform logic and validity checks on those statements.

The above is all the content of the article "what is the use of DataAdapter classes in VB.NET database". Thank you for reading! Hope to share the content to help you, more related knowledge, welcome to follow the industry information channel!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.