Data Models

A data model is a description of the structure of a database. Data models generally, fall into 3 categories according to the level of the description.

High-level ( or conceptual) data models closely approximate the miniworld.
Representational ( or, implementational ) data models are intermediate data models close to the miniworld but also reflecting the actual organization of data in the database.
Low-level ( or, physical) data models describe details of physical storage, generally transparent to casual or parametric end users.

A data model is often specified by a database schema typically displayed in a schema diagram that consists of schema constructs.

Database State

The actual data in a data base at a particular instant is the database state, which consists of a set of instances for each schema construct.
Defining a database consists of specifying a schema to the DBMS. We then have a database in an empty state, with no data.
When data is first loaded, the database is in its initial state.
Subsequently, each update creates another state. The DBMS must guarantee that each such state is a valid state that satisfies schema specifications.
The schema is the intension, while a database state is the extension of the schema.

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