Database Systems: Concepts, Design and Applications

2.2. Schemas, Subschema and Instances

When the database is designed to meet the information needs of an organisation, plans (or scheme) of the database and actual data to be stored in it becomes the most important concern of the organisation. It is important to note that the data in the database changes frequently, while the plans remain the same over long periods of time (although not necessarily forever). The database plans consist of types of entities that a database deals with, the relationships among these entities and the ways in which the entities and relationships are expressed from one level of abstraction to the next level for the users’ view. The users’ view of the data (also called logical organisation of data) should be in a form ...