Datasheet

Figure 1-1

Data Access Consumers

Developers who use languages that support pointers — such as C, C++, VJ++, and so on — can speak

directly to the ODBC and OLE-DB APIs. However, developers using a language such as Visual Basic

need another layer. This is where the data access consumers such as DAO, RDO, ADO, and ADO.NET

come into play.

DAO

With the release of Visual Basic 2.0, developers were introduced to a new method for accessing data,

known as Data Access Objects (DAO). This was Microsoft’s first attempt to create a data consumer API.

Although it had very humble beginnings, and when first released only supported forward-only opera-

tions against ODBC data sources, it was the beginning of a series of libraries that would lead developers

closer to the ideal of Universal Data Access. It also helped developers using higher-level languages such

as Visual Basic to take advantage of the power of ODBC that developers using lower-level languages

such as C were beginning to take for granted.

DAO was based on the JET engine, which was largely designed to help developers take advantage of the

desktop database application Microsoft was about to release, Microsoft Access. It served to provide

another layer of abstraction between the application and data access, making the developer’s task sim-

pler. Although the initial, unnamed release with Visual Basic 2.0 only supported ODBC connections, the

release of Microsoft Access 1.0 marked the official release of DAO 1.0, which supported direct communi-

cation with Microsoft Access databases without using ODBC. Figure 1-2 shows this relationship.

DAO 2.0 was expanded to support OLE-DB connections and the advantages that come along with it. It

also provided a much more robust set of functionality for accessing ODBC data stores through the JET

engine. Later, versions 2.5 and 3.0 were released to provide support for ODBC 2.0 and the 32-bit OS

introduced with Windows 95.

Application

OLE-DB

Consumer

Data Store

OLE-DB

Provider

Data Source

Specific API

Chapter 1

03_584375 ch01.qxd 10/28/05 10:49 PM Page 6