Visual Basic.NET How to Program (PDF)

Chapter 19 Database, SQL and ADO .NET 923
to be displayed (line 47)—a table name, in this case. Readers can try entering their own queries
in the text box and then pressing the Submit Query button to execute the query.
Fig. 19.28 displays the output for FrmDisplayQueryResults. The first screenshot
demonstrates the query results of retrieving all records from the Authors table. As
the second screen capture demonstrates, clicking any column sorts the rows according to
the contents of that column in either ascending or descending order.
19.7 Programming with ADO .NET: Modifying a DBMS
Our next example implements a simple address-book application that enables users to insert
records into, locate records from and update the Microsoft Access database
Addressbook.
The Addressbook application (Fig. 19.29) provides a GUI through which users can
execute SQL statements on the database. Earlier in the chapter, we presented examples
explaining the use of SELECT statements to query a database. This example provides that
same functionality.
Event handler cmdFind_Click (lines 72–119) performs the SELECT query on the
database for the record associated with the String entered in txtLast. This represents
the last name of the person whose record the user wishes to retrieve. Line 81 invokes
method Clear of class DataSet to empty the DataSet of any prior data. Lines 85–87
modify the text of the SQL query to perform the appropriate SELECT operation. The Ole-
DbDataAdapter method Fill then executes this statement (line 91). Notice how a different
overload of method Fill from the previous example has been used in this situation.
Only the DataSet to be filled is passed as an argument. Finally, the TextBoxes are
updated with a call to method Display (line 94).
Methods cmdAdd_Click (lines 122–173) and cmdUpdate_Click (lines 176–
232) perform INSERT and UPDATE operations, respectively. Each method uses members
of class OleDbCommand to perform operations on a database. The instance properties
InsertCommand and UpdateCommand of class OleDbDataAdapter are instances
of class OleDbCommand.
1 ' Fig. 19.29: AddressBook.vb
2 ' Using SQL statements to manipulate a database.
3
4 Imports System.Windows.Forms
5
6 Public Class FrmAddressBook
7 Inherits Form
8
9 ' top set of command buttons
10 Friend WithEvents cmdFind As Button
11 Friend WithEvents cmdAdd As Button
12 Friend WithEvents cmdUpdate As Button
13 Friend WithEvents cmdClear As Button
14 Friend WithEvents cmdHelp As Button
15
16 ' textbox identifier labels
17 Friend WithEvents lblId As Label
Fig. 19.29 Database modification demonstration (part 1 of 9).