Chapter 2 - Database System Concepts and Architecture

Data Models, Schemas and Instances

A characteristic of the database approach is that it provides a level of data abstraction, by hiding details of data storage that are not needed by most users.

A data model is a collection of concepts that can be used to describe the structure of a database. The model provides the necessary means to achieve the abstraction.

The structure of a database is characterized by data types, relationships, and constraints that hold for the data. Models also include a set of operations for specifying retrievals and updates.

Data models are changing to include concepts to specify the behaviour of the database application. This allows designers to specify a set of user defined operations that are allowed.

Categories of Data Models

Data models can be categorized in multiple ways. • High level/conceptual data models – provide concepts close to the way users perceive the data. • Physical data models – provide concepts that describe the details of how data is stored in the computer. These concepts are generally meant for the specialist, and not the end user. • Representational data models – provide concepts that may be understood by the end user but not far removed from the way data is organized.

Conceptual data models use concepts such as entities, attributes and relationships. • Entity – represents a real world object or concept • Attribute - represents property of interest that describes an entity, such as name or salary. • Relationships – among two or more entities, represents an association among two or more entities.

Representational data models are used most frequently in commercial DBMSs. They include relational data models, and legacy models such as network and hierarchical models.

Physical data models describe how data