UNIT 3: INSTANCES AND SCHEMES

1. Concepts of Instances and Schemes

The information in a database at a particular point in time is called an instance of the database. While the overall design of the database is called the database scheme.

 Link between Instances and Schemes

In order to grasp the key aspects of Instances and Schemes we would identify the link between these two concepts. Instances and Schemes are two terms closely associated with the mode of change database over time.

 Analogy with Programming Languages

 Instances and Schemes correlate with programming languages as follows:

•          Data type definition - scheme

•          Value of a variable - instance

 Categories of Schemes

There are several classes of schemes, corresponding to the levels of abstraction: 

•          Physical scheme

•          Conceptual scheme

•          Sub scheme (can be many)