After inventing of DBMS, several models are proposed stating various ways of representing data within the database and their advantages, called as data models. These data models are classified into three categories based on their purpose as follows.
- Physical Data Models
- Conceptual Data Models
- Logical Data Models
Physical
Data Models : The
data models that concentrate on how the data is physically stored within the
database are called as physical data models.
Conceptual
Data Models : The
data models that concentrate on how the data and relationship between data can
be represented diagrammatically are called as conceptual data models. One of
the most popular conceptual model is Entity-Relationship
Model(E-R Model).
E-R
model uses the terminology Entity, Attribute and Relationship. An entity is a
real world object stored within the database. If you store student information
in the database, then student is called an entity, if you store employee
information within the database, then employee is called an entity and so on.
An attribute is a characteristic feature of the entity. For student entity sid,
sname, course, address all these are called as attributes. A relationship is an
association between entities. Relationship between entities are classified into
one to one(1:1), one to many(1:M) and
many to many(M:N).
ER Diagrams:
Within
E-R diagrams an entity is represented with a rectangle, an attribute with oval
shape and relationship with diamond shape etc.
The E-R diagram for the entities
Student and Course is as follows. The relationship type between course and
student is one to many. This is because one student can join only one course
and a course can be chosen by any number of students.
Logical
Data Models : The
data models that concentrate on what data is stored in the database and what
type of relationship exist between data are called as logical data models. The
following lists of logical data models are available.
1.
Hierarchical Data Model
2.
Network Data Model
3.
Relational Data Model
4.
Object Data Model
5.
Object Relational Data Model
Hierarchical
Data Model : in
this data model data is organized hierarchically like in tree data structure.
Within hierarchical database model it is not possible to represent many to many
relationships. This drawback leads to the development of network data model.
Student information can be represented in hierarchical data model as follows.
Network
Data Model : in
this data model also data is represented same as in case of hierarchical data
model except that it can represent many to many relationship. Student
information is represented in network data model as follows.
In
both hierarchical and network data models, organization of data is very
difficult as it is organized like a tree data structure. Especially deleting a
record will be very complex process as all records that are based on deleted
record must be rearranged. These drawbacks lead to the development of
relational data model.
Relational
Data Model : Relational
data model was proposed by E.F.Codd
in 1970. He proposed the model
theoretically and first practical implementation of relational model was System-R and is developed by IBM Corporation. In relational data
model data is organized in the form of attributes,
tuples and relations. Attribute,
tuple and relation can be compared to a field, record and file in file
management system. These are technical terms given by E.F. Codd and now a days
these terms are replaced with column, row and table respectively.
Object Data
Model : This data model is based on object oriented concepts of
programming. This model provides OOP features within the database. This model
also becomes popular as it has advantages of OOP. Examples are Object Store, Cactus etc.,
Object
Relational Data Model : this
model combines the features of object data model with the features of
relational data model. Hence this model has both the advantages of relational
data model as well as object data model. Example is Oracle 8.0 onwards.
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