DBMS-Data Models
A data model—a collection of concepts that can be used to describe the structure of a database—provides the necessary means to achieve this abstraction. By structure of a database we mean the data types, relationships, and constraints that should hold on the data.
Categories of Data Models
Many data models have been proposed, and we can categorize them according to the types of concepts they use to describe the database structure. High-level or conceptual data models provide concepts that are close to the way many users perceive data, whereas low-level or physical data models provide concepts that describe the details of how data is stored in the computer. Concepts provided by low-level data models are generally meant for computer specialists, not for typical end users. Between these two extremes is a class of representational (or implementation) data models, which provide concepts that may be understood by end users but that are not too far removed from the way data is organized within the computer. Representational data models hide some details of data storage but can be implemented on a computer system in a direct way.
Conceptual data models use concepts such as entities, attributes, and relationships. An entity represents a real-world object or concept, such as an employee or a project, that is described in the database. An attribute represents some property of interest that further describes an entity, such as the employee’s name or salary. A relationship among two or more entities represents an interaction among the entities; for example, a works-on relationship between an employee and a project. Entity-Relationship model—a popular high-level conceptual data model.
Representational or implementation data models are the models used most frequently in traditional commercial DBMSs, and they include the widely-used relational data model, as well as the so-called.
legacy data models—the network and hierarchical models—that have been widely used in the past. Representational data models represent data by using record structures and hence are sometimes called record-based data models.
We can regard object data models as a new family of higher-level implementation data models that are closer to conceptual data models. . Object data models are also frequently utilized as high-level conceptual models, particularly in the software engineering domain.
Physical data models describe how data is stored in the computer by representing information such as record formats, record orderings, and access paths. An access path is a structure that makes the search for particular database records efficient.
