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A description of data in terms of a data model is called the schema
Every relation has a schema, which describes the name of the relation, name of each attribute (field or column), and the type of each column. e.g. Students(sid: string, name: string, login: string,age: integer, gpa: real)
Schemas versus 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).