What is data model (hierachical,network and relational)

Ø  Data model –The data model is fundamental representing information. The data model determine what data representation mechanism are supported by the DBMS. The data definition language is just the specific set of language construct available to describe an actual applications data in term of the data model.

OR

A Database model defines the logical design and structure of a database and defines how data will be stored, accessed and updated in a database management system. While the Relational Model is the most widely used database model.

Types of database models - There are many kinds of data models. Some of the most common ones include:

·         Hierarchical database model

·         Relational model

·         Network model

·         Object-oriented database model

·         Entity-relationship model

·         Document model

·         Entity-attribute-value model

·         Star schema

·         The object-relational model, which combines the two that make up its name

1.      Hierarchical model –

·        This database model organises data into a tree-like-structure, with a single root, to which all the other data is linked.

·        The hierarchy starts from the Root data, and expands like a tree, adding child nodes to the parent nodes.

·        In this model, a child node will only have a single parent node.

·        In hierarchical model, data is organised into tree-like structure with one one-to-many relationship between two different types of data, for example, one department can have many courses, many professors and of-course many students.

2.      Network model –

·        This is an extension of the Hierarchical model.

·         In this model data is organized more like a graph, and are allowed to have more than one parent node.

·        In this database model data is more related as more relationships are established in this database model.

·         This database model was used to map many-to-many data relationships.

·        This was the most widely used database model, before Relational Model was introduced.

3.      Relational Model –

·        In this model, data is organized in two-dimensional tables and the relationship is maintained by storing a common field.

·        The basic structure of data in the relational model is tables.

·         All the information related to a particular type is stored in rows of that table.

·        Hence, tables are also known as relations in relational model.

extended feature of er diagram generalization,specialization,aggregation

Ø  Extended feature of ER diagram –

1.      Generalization

2.      Specialization

3.      Aggregtion

1)     Generalization –

·        The process of generalizing entities, where the generalized entities contain the properties of all the generalized entities, is called generalization.

·        Generalization is opposite of specialization (reverse of specialization).

·        In generalization there exists a bottom up approach.

·         In generalization, a number of entities are brought together into one generalized entity based on their similar characteristics.

·        Employee and customer can be a person so we are removing some attributes.

·        Lower level entities are producing higher level entity.

v Inherit – inherit is an important feature of generalization, in generalization it allow upper level entity to inherit lower level entity attribute.

2)     Specialization –

·        The process of classifying an entity into more specialized entity.

·        Specialization is the just opposite of generalization.

·         In specialization a group of entity is divided into sub group on their characteristic.

·        It is a top-down approach in which one higher level entity can be broken down into two lower level entities.

·         In specialization, a higher level entity may not have any lower-level entity sets, it's possible.

v Inherit – inherit is an important feature of specialization, in specialization it allow lower level entity to inherit upper level entity attribute.

          

3)     Aggregation – A feature of entity relationship model that allows a relationship set to participate in another relationship set. This is indicated on an ER diagram by drawing a dashed box around the aggregation.

·        The meaning of aggregation is way of abstraction.

·         The process through which one can treat the relationship as higher level entities is known as aggregation.

·        It shows relationship among relationship