Definition : A data model is a collection of conceptual tools for describing data, their relationships, data semantics and consistency constraints.
(1) Record Based Logical Models
- Physical Data Models
- Object Based Logical Models
- ER model
- Object Oriented Model
(1) Hierarchical Model
- It organises data in a tree structure.
- There is a hierarchy of a parent and child data segments. This segment implies that a record can have repeating information, generally in the child data segment.
- Here the data and relationships among data are represented by records and links
The record are organized as collection of trees rather than arbitrary graphs.
(2) Relational Model
(3) Network Model
— • Data in the Network model are represented by collection of records and relationships among data are represented by links which can be viewed as pointers.
- The records in the database are organized as collection of arbitrary graphs.
- It permits M : N relationship.
(4) Object-Based Logical Model
They are used in describing data at the conceptual and view levels. examples are :
- E-R Model
- Object-oriented Model.
(a) E-R-Model :
- it is based on the perception of a real world which consists of a collection of basic objects called entities and relationships among these objects.
- An entity is an object that is distinguishable from other objects by a specific set of attributes.
- A relationship is an association among several entities.
- The set of all entities of same type and relationships of same type are termed as entity set and relationship set respectively.
The overall logical structure of database can be expressed graphically by an E-R-diagram.
(b) Object-Oriented Model
- It is based on a collection of objects.
- And object has :
- A set of variables that contains the data for object. The value of each variable is itself an object.
- A set of messages to which an object responds.
- A method, which is a body of code to implement each message. A method returns a value as the response to the message.
The objects that contain the same type of values and same methods are grouped together into classes. A class may be viewed as a type definition for objects.
(3) Physical Data Models
- Physical data models are used to describe data at the lowest level.