Instances and Schemes

1. Databases change over time.
2. The information in a database at a particular point in time is called an instance of the database.
3. The overall design of the database is called the database scheme.
4. Analogy with programming languages:
   • Data type definition - scheme
   • Value of a variable - instance
5. There are several schemes, corresponding to levels of abstraction:
   • Physical scheme
   • Conceptual scheme
   • Subscheme (can be many)