- A data model is a collection of conceptual tools for describing data, data relationships, data semantics, and consistency constraints.
- Object Based Data Models
- Physical Data Models
- Record Based Data Models
- The object based and record based data models are used to describe data at the conceptual and external levels, the physical data model is used to• describe data at the internal level.
Object Based Data Models
- Object based data models use concepts such as entities, attributes, and relationships. An entity is a distinct object (a person, place, concept, and event) in the organization that is to be represented in the database.
- An attribute is a property that describes some aspect of the object that we wish to record, and a relationship is an association between entities.
- Some of the more common types of object based data model are:
- Object Oriented
- The Entity-Relationship model has emerged as one of the main techniques for modeling database design and forms the basis for the database design methodology.
- The object oriented data model extends the definition of an entity to include, not only the attributes that describe the state of the object but also the actions that are associated with the object, that is, its behavior.
- The object is said to encapsulate both state and behavior. Entities in semantic systems represent the equivalent of a record in a relational system or an object in an OO system but they do not include behavior (methods).
Physical Data Models
- Physical data models describe how data is stored in the computer, representing information such as record structures, record ordering, and access paths.
- There are not as many physical data models as logical data models, the most common one being the Unifying Model.
Record Based Logical Models
- Record based logical models are used in describing data at the logical and view levels.
- In contrast to object based data models, they are used to specify the overall logical structure of the database and to provide a higher-level description of the implementation.
- Record based models are so named because the database is structured in fixed format records of several types. Each record type defines a fixed number of fields, or attributes, and each field is usually of a fixed length.
- The three most widely accepted record based data models are:
- Hierarchical Model
- Network Model
- Relational Model
- The relational model has gained favor over the other two in recent years. The network and hierarchical models are still used in a large number of older databases.