how to generate barcode in c#.net with example Issuing Database Commands in Visual Studio .NET

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Issuing Database Commands
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As shown in the preceding code snippets, you can perform insert, update, and delete operations through a Command object by means of the ExecuteNonQuery method, which returns the number of records that were affected by the statement:
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Dim sql As String = _ INSERT INTO Authors (Author, [Year Born]) VALUES ( Joe Doe , 1955)" Dim cmd As New OleDbCommand(sql, cn) Run the query; get the number of affected records. Dim records As Integer = cmd.ExecuteNonQuery()
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Of course, you can update existing records by using the UPDATE SQL statement and delete existing records with the DELETE statement. There isn t much else to say about this method except that as with all database operations you should protect it with a Try block, and ensure that you dispose of the Command object and close the connec; tion in the Finally block:
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Try Run the query; get the number of affected records. Dim records As Integer = cmd.ExecuteNonQuery() Catch ex As Exception Process the error here.
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Finally cmd.Dispose() cn.Close() End Try
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ADO.NET in Connected Mode
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Reading Data
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You can read data from a data source in three ways: by using the ExecuteReader method and the DataReader object to read complete resultsets; by using the ExecuteScalar method to read individual values; or by using the ExecuteXmlReader method and the XmlReader object to read the results of a FOR XML query on a SQL Server 2000 data source. (The ExecuteXmlReader method is exposed only by the SqlConnection object.)
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Using the ExecuteReader Method
The most common way to query the database in connected mode is through the Exe; cuteReader method of the Command object. This method returns a DataReader object, which you then use to read the resultset one row at a time, as you d do with a forwardonly, read-only recordset under classic ADO. There are actually different versions of this object: OleDbDataReader, SqlDataReader, OdbcDataReader, and OracleDataReader.
Create a query command on the connection.
Dim cmd As New OleDbCommand( SELECT * FROM Publishers", cn)
Run the query; get the DataReader object.
Dim dr As OleDbDataReader = cmd.ExecuteReader()
Read the names of all the publishers in the resultsets.
Do While dr.Read()
Debug.WriteLine(dr.Item( Name )) Loop Close the DataReader and dispose of the Command. dr.Close() cmd.Dispose()
I ll discuss the DataReader object and its methods in greater detail in the section The DataReader Object later in this chapter. For now, I ll focus on how you can affect the query by passing an optional CommandBehavior bit-coded value to the ExecuteReader method. The available values for this argument are
CloseConnection The connection should be closed immediately after the DataReader object is closed. SingleRow The SQL statement is expected to return a single row of data. The OLE DB .NET Data Provider uses this information to optimize the data retrieval operation. SingleResult The SQL statement is expected to return a single resultset. This value ensures that only a resultset is returned, even if the SQL query contains mul; tiple SELECT statements.
Part V:
Database Applications
KeyInfo The query returns column and primary key information and is exe; cuted without locking the selected rows. In this case, the SQL Server .NET Data Provider appends a FOR BROWSE clause to the SQL statement, which requires that the table have a time-stamp field and a unique index. (See SQL Server Books Online for additional information.) SequentialAccess The query results are read sequentially at the column level instead of being returned as a whole block to the caller. You should use this option when the table contains very large text and binary fields that you read in chunks using the GetChars and GetBytes methods of the DataReader object, as I ll explain in the Working with BLOB Columns section, later in this chapter. SchemaOnly The query returns column information only and doesn t affect the database state.
The CloseConnection value is especially useful when you create a function that opens a connection and returns a DataReader object. Because the caller doesn t have a reference to the Connection object, it can t close it, but the CloseConnection argument ensures that the connection is closed automatically when the caller closes the DataReader:
Function GetReader(ByVal connString As String, ByVal sql As String) _ As OleDbDataReader Dim cn As New OleDbConnection(connString) cn.Open() Dim cmd As New OleDbCommand(sql, cn) GetReader = cmd.ExecuteReader(CommandBehavior.CloseConnection) cmd.Dispose() End Function
The SingleRow option is useful when you re absolutely sure that the resultset contains only one row. This is often the case when the WHERE clause of the query filters a sin; gle record by its primary key, as in this example:
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