Friday, June 8, 2012
Schema VS Instances
Schema VS Instances
Database Schema: The description of a database. Includes descriptions of the database structure and the constraints that should hold on the database.
Schema Diagram: A diagrammatic display of (some aspects of) a database schema.
Schema Construct: A component of the schema or an object within the schema, e.g., STUDENT, COURSE.
Database Instance: The actual data stored in a database at a particular moment in time. Also called database state(or occurrence).
Database Schema Vs. Database State
Database State: Refers to the content of a database at a moment in time.
Initial Database State: Refers to the database when it is loaded
Valid State: A state that satisfies the structure and constraints of the database.
Distinction
The database schema changes very infrequently. The database state changes every time the database is updated.
Schema is also called intension, whereas state is called extension
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