Sams Teach Yourself Visual Basic 6 in 24 Hours
- Table of Contents
- Copyright
- About the Author
- Acknowledgments
- Introduction
- Who Should Read This Book
- What This Book Will Do for You
- Can This Book Really Teach Visual Basic in 24 Hours?
- What You Need
- Files on the Visual Basic Distribution CD-ROM
- Conventions Used in This Book
- Enough! Time Is Ticking!
- Part I: Introducing Visual Basic
- Hour 1. Visual Basic at Work
- Hour 2.Analyzing Visual Basic Programs
- Hour 3.Controls and Properties
- Hour 4.Examining Labels, Buttons, and Text Boxes
- Part II: Coding the Details
- Hour 5.Putting Code into Visual Basic
- Hour 6.Message and Input Boxes
- Hour 7.Making Decisions
- Hour 8.Visual Basic Looping
- Part III:Putting Code to Work
- Hour 9.Combining Code and Controls
- Hour 10.List Boxes and Data Lists
- Hour 11.Additional Controls
- Hour 12.Dialog Box Basics
- Part IV:Programming with Data
- Hour 13.Modular Programming
- Hour 14.Built-In Functions Save Time
- Hour 15.Visual Basic Database Basics
- File Concepts
- Introduction to Database Processing
- The Data Form Wizard
- Summary
- Q&A
- Workshop
- Hour 16.Printing with Visual Basic
- Part V:Sprucing Up Programs
- Hour 17.Menus and Visual Basic
- Hour 18.The Graphic Image Controls
- Hour 19.Toolbars and More Graphics
- Hour 20.Writing Correct Applications
- Part VI:Advancing Visual Basic Applications
- Hour 21.Visual Basic and ActiveX
- Hour 22.Object Basics
- Hour 23.Distributing Your Applications
- Hour 24.Online Visual Basic
- Part VII:Appendixes
- Appendix A.Operator Precedence
- Appendix B.Answers
- Appendix C.Using the CD-ROM
Introduction to Database Processing
A database system is a program that organizes, manipulates, retrieves, and reports data.
If you use a database for your data, you can take advantage of Visual Basic's Data control to access the database from within your Visual Basic applications. The Data control makes it easy for you to retrieve data and display values from a database file without using any of Visual Basic's specific file-related commands that you learned about in the first part of this lesson.
The Data Control
If you use any of the following database applications, you'll be able to write a Visual Basic application that accesses the data within your database without resorting to the file-related commands described earlier in this lesson: Microsoft Access, dBASE, Excel, FoxPro, Lotus, Paradox, and text-based data files.
The Data control makes database access simple.
A field is a column of data inside a file.
A database application manages your data in a record and field format. The database, however, doesn't necessarily store your data in records and fields in a table-like format, but the database makes the data appear to your program in that format. Visual Basic takes advantage of this format and retrieves data in the record and field format no matter how the database physically stores the data.
One challenge when using database access is that you must often describe parts of the database to Visual Basic. Visual Basic cannot magically understand your database structure. When you place the Data control on your form, you'll have to tell the control the structure of your data and tell the Data control which parts of the data to access so that the control can properly retrieve data. For example, by setting appropriate property values, you must tell the Data control the name of your database, the table, and the fields to access.
A table is a logical collection of data in a database. A database might contain several tables.
Some databases, such as Microsoft Access, store all the related database files in a single global file called the database file. Inside the database, the individual groups of records and fields are called tables. Other database systems, such as dBASE, keep track of a database's data in multiple files. When you use a database such as Microsoft Access, as this lesson does, you must describe both the overall database and the individual table name within the database that the Data control is to use.
This lesson doesn't provide you with a table of Data control property descriptions because too many of the descriptions are database related and too advanced for this discussion. You might not need to know more properties than described in the small data application at the end of this lesson in most cases. You'll probably be surprised at the amount of power the Data control gives you.
Figure 15.1 illustrates a Data control in use. Notice that the Data control works much like a VCR's series of buttons. You can step through the database one record at a time. The control itself doesn't display data. Instead, the control simply lets you regulate the access of data. You'll have to supply other controls, such as labels or text boxes, to display and collect data for the database. In other words, if you want the user to be able to move to a particular record, you'll supply a Data control that the user can push to get the record in the database. Then your application can display the data on the form using label controls.
A bound control is a control you can link to a database, via the Data control, that displays and updates database records if the user modifies the data in the bound control.
The Data control is a two-way street; not only does it display database data, but your user can modify the data that the Data control displays, and the Data control makes sure that the changes are made to the underlying database through bound controls. If you don't want the user to be able to change data displayed from a Data control, you can use a label and not a text box to display the database data. You can bind several other controls to the Data controls and make the control read-only so that the user cannot change the underlying database.
A Simple but Powerful Application
Figure 15.1's text boxes are bound to the Data control on the form. Therefore, if the user changes the data in any text box that displays a value from the database, the underlying record's field value changes as well. The application is required to do nothing.
Figure 15.1 shows an application that you are about to build. Do you want to be really shocked? The application will contain no code whatsoever. The entire database access and update can be done just with the controls on the form!
Figure 15.1's book publisher application uses a database that comes with Visual Basic named Biblio.mdb. The database is a Microsoft Access database and contains computer book titles and publishers.
Perform these steps to build Figure 15.1's application:
- Create a new project and name the form frmData. Add a caption that says Database Sample. Resize the form to a Height property of 5775 and a Width property of 7170.
- Add a label with these properties:
Name: lblAnnounce Alignment: 2-Center BorderStyle: 1-Fixed Single Caption: Computer Book Publishers Font: Bold 14 points Height: 855 Left: 2160 Top: 360 Width: 2895
- Add five additional field labels as follows:
-
Field 1
Name: lblPub Alignment: 1-Right Justify Caption: Publisher: Font: 12 points Left: 720 Top: 1800 Width: 1215 -
Field 2
Name: lblAddress Alignment: 1-Right Justify Caption: Address: Font: 12 points Left: 720 Top: 2400 Width: 1215 -
Field 3
Name: lblCity Alignment: 1-Right Justify Caption: City: Font: 12 points Left: 840 Top: 3000 Width: 495 -
Field 4
Name: lblState Alignment: 1-Right Justify Caption: State: Font: 12 points Left: 3360 Top: 3000 Width: 735 -
Field 5
Name: lblZip Alignment: 1-Right Justify Caption: Zip: Font: 12 points Left: 4800 Top: 3000 Width: 495
-
Field 1
- Before adding the text boxes, add the Data control. To begin, double-click the Data control to send the control to the center of the form. Change these property values:
Name: dtaBiblio Caption: Click to control data display Left: 1920 Top: 4200 Width: 3255
The text that appears in the center of the Data control is the caption—never data. The Data control doesn't display data. Instead, it regulates the display of data, and you use other fields to hold the displayed data. The text boxes that you place in the next two steps will display the data. You'll bind the text boxes to the Data control, and the Data control will be connected to the database. You must now make that connection; click the DatabaseName property, and then click the ellipsis that appears. Select the Biblio.mdb database from VB's folder (you might have to locate the folder from the file list that appears). Now that the Data control is connected to the database, you can add the text boxes. The database named Biblio.mdb, now connected to the dtaBiblio Data control, contains several tables. Therefore, not only must you tell the Data control which database to use, you must also specify the table source for the data. Select Publishers from the RecordSource property's drop-down list box. The Data control will now produce records only from Biblio.mdb's Publishers table. - Add a text box to the form with these properties:
Name: txtPublisher BackColor: (select the ToolTip color) BorderStyle: 1-Fixed Single Left: 2040 Top: 1800 Width: 4215
Now you must inform the text box that its data source is the Data control named dtaBiblio. Set the DataSource property to dtaBiblio by selecting dtaBiblio from the DataSource property's pull-down list box. (dtaBiblio is the only item that appears, but if the form contained additional Data controls, you would have to select the proper one for the text box's data source.) The Publishers table, controlled by the Data control, contains several fields. Therefore, not only must you tell the text box which Data control to connect to, but you must also specify the field. Change the DataField property to Company Name. Company Name is the database's field name that holds the publisher name. Set Font to 12. - Add the following four text box controls:
-
Control 1
Name: txtAddress BackColor: ToolTip yellow BorderStyle: 1-Fixed Single DataField: Address DataSource: dtaBiblio Font Size: 12 Left: 2040 Top: 2400 Width: 4215 -
Control 2
Name: txtCity BackColor: ToolTip yellow BorderStyle: 1-Fixed Single DataField: City DataSource: dtaBiblio Font Size: 12 Left: 1440 Top: 3000 Width: 1815 -
Control 3
Name: txtState BackColor: ToolTip yellow BorderStyle: 1-Fixed Single DataField: State DataSource: dtaBiblio Font Size: 12 Left: 4200 Top: 3000 Width: 555 -
Control 4
Name: txtZip BackColor: ToolTip yellow BorderStyle: 1-Fixed Single DataField: Zip DataSource: dtaBiblio Font Size: 12 Left: 5400 Top: 3000 Width: 1055
-
Control 1
- Add a separating line with the Line control that has these properties:
X1: 0 X2: 7080 Y1: 3720 Y2: 3720
Run the application, and you'll be looking at the first record in the database. Click the Data control's buttons to move through the database records. If you change a value, you'll be changing the actual database itself because of the bound text box controls. Despite the fact that the application requires a lot of controls, no code is required because of the Data control's powerful database retrieval and update capabilities.
The Data Form Wizard | Next Section
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