Visual Basic 6 Black Book
(Publisher: The Coriolis Group)
Author(s): Steven Holzner
ISBN: 1576102831
Publication Date: 08/01/98

ADO

As we saw in the last chapter, ActiveX Data Objects (ADO) access data from OLE DB providers. The Connection object is used to specify a particular provider and any parameters. To connect to a data source, you use a Connection object. Using that connection, you can create a new record set, and using the Recordset object’s methods and properties, you can work with your data.

An ADO transaction marks the beginning and end of a series of data operations that are executed across a connection. ADO makes sure that changes to a data source resulting from operations in a transaction either all occur successfully, or not at all. If you cancel the transaction or one of its operations fails, then the result will be as if none of the operations in the transaction had occurred.

In this chapter, we’ll see how to create connections using the ADO Connection object and how to open data providers, creating an ADO Recordset object. We’ll read data from the data provider and see how to display and modify it. In fact, we’ll see how to support data-bound controls directly in code.

Although the ADO model is a complex one, and OLE DB is even more complex, we’ll see that many of the core ADO Resultset methods are the same as the DAO Resultset methods.

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TIP:  Note that in DAO and ADO you work with record sets, and in RDO with result sets; it’s very easy to confuse the terminology here.

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That’s it, then, for the overview of databases. We’ve seen how the process works in overview; now it’s time to turn to the Immediate Solutions.

Immediate Solutions

A Full-Scale DAO Example

To illustrate DAO data handling in code, we’ll build a fully functional DAO project—the daocode project. This program has a File menu with the following items:

•  New Database—Creates a new database.

•  Open Database—Opens a database.

•  Close Database—Closes the current database.

•  New Table—Creates a new table.

•  Search—Searches the database.

•  Sort—Sorts the database.

•  Exit—Exits the application.

Using The Daocode Example To Create And Edit A Database

To create a database file, select the New Database menu item. Next, add a table to that database with the New Table menu item, then add records to that table. When you’re ready to store the database on disk, use the Close Database item.

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WARNING!  If you don’t create a table in a database before trying to add data to a table in that database with the Add or Edit buttons, the daocode program generates an error.

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In addition, the program has buttons that let users add, edit, update, and delete records, as well as letting them move through a database, as shown in Figure 25.1. Each time you want to add a record (including when you enter the first record of a new database), click the Add New Record button, type in the data for the record’s fields, and click the Update Database button to update the database.

Figure 25.1  Our DAO database-building application, the daocode project.

To edit a record, open the record, click the Edit button, edit the data in the record’s fields, and click the Update Database button to update the database. For simplicity, this program only creates tables with two fields, although you can place as many records as you like in each table.

We’ll develop the code for this example program in the next several topics of this chapter. For reference, the main form of this example program is located in the daocode folder on this book’s accompanying CD-ROM; the form the user uses to specify the names of the fields in a new table is located in the TableForm folder on CD-ROM; and the code for the form in which the user can enter a text string to search for is located in the SearchForm folder on the CD-ROM.