A Database model defines the logical design of data. The model describes the relationships between different parts of the data. In history of database design, three models have been in use.
It refers to a set of concepts to describe the structure of a database and certain constraints that the database should obey.
Example of Hierarchical Movel
Example of Network Model
Categories of data models
- Conceptual (high-level, semantic) data models: Provide concepts that are close to the way many users perceive data. (Also called entity-based or object-based data models.)
- Physical (low-level, internal) data models: Provide concepts that describe details of how data is stored in the computer.
- Implementation (representational) data models: Provide concepts that fall between the above two, balancing user views with some computer storage details.
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.
Database Instance: The actual data stored in a database at a particular moment in time. Also called database state (or occurrence).
- Hierarchical Model
In this model each entity has only one parent but can have several children . At the top of hierarchy there is only one entity which is called Root.
- Network Model:
In the network model, entities are organised in a graph, in which some entities can be accessed through several path
- Relational Model :
In this model, data is organised in two-dimesional tables called relations. The tables or relation are related to each other.
Entity And Entity Sets:
An entity is a basic unit of E-R model which is an object or a thing in real world having independent existence. An entity may be concrete and a physical existence (e.g.: person, place) or it can be abstract or conceptual existence like loan, course.
Entity set is a collection of entities of a particular entity type at any point of time.