Part-2 : Database Model, Entity-Relationship model and Different Relationships

By | February 8, 2016

Database Model

Logical design of data describes the relationship between different parts of the data. Data models define how the logical structure of a database is modeled. Data Models are fundamental entities to introduce abstraction in a DBMS. Data models define how data is connected to each other and how they are processed and stored inside the system.
The very first data model could be flat data models, where all the data used are to be kept in the same plane. Earlier data models were not so scientific, hence they were prone to introduce lots of duplication and update anomalies.

Entity-Relationship Model

Entity-Relationship (ER) Model is based on the notion of real-world entities and relationships among them. While formulating real-world scenario into the database model, the ER Model creates entity set, relationship set, general attributes and constraints.
ER Model is best used for the conceptual design of a database.
ER Model is based on − Entities and their attributes.
Relationships among entities.
These concepts are explained below.
Entity − An entity in an ER Model is a real-world entity having properties called attributes. Every attribute is defined by its set of values called domain. For example, in a school database, a student is considered as an entity. The student has various attributes like name, age, class, etc.
Relationship − The logical association among entities is called relationship. Relationships are mapped with entities in various ways. Mapping cardinalities define the number of association between two entities.

Mapping cardinalities

  • one to one
  • one to many
  • many to one
  • many to many

E-R Diagram

ER-Diagram is a visual representation of data that describes how data is related to each other.

Symbols & Notations

The symbols it include in the Entity-Relationship diagram are,
  • Entity
  • Attribute
  • Week Entity
  • Key Attribute
  • Relationship
  • Week Key Attribute
  • Multivalued Attribute
  • Composite Attribute


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Relationship

A Relationship describes relations between entities. The relationship is represented using diamonds. There are three types of relationship that exist between Entities.
  • Binary Relationship
  • Recursive Relationship
  • Ternary Relationship

Binary Relationship

Binary Relationship means the relation between two Entities. This is further divided into three type
  • One-to-one
  • One-to-many
  • Many-to-many

Recursive Relationship

When an Entity is related with itself it is known as Recursive Relationship.

Ternary Relationship

Relationship of degree three is called Ternary relationship.

Generalization Relationship

A generalization is a bottom-up approach in which two lower level entities combine to form a higher level entity. In generalization, the higher level entity can also combine with other lower level entity to make further higher level entity.

Specialization Relationship

Specialization is opposite to Generalization. It is a top-down approach in which one higher level entity can be broken down into two lower level entity. In specialization, some higher level entities may not have lower-level entity sets at all.

Aggregation Relationship

Aggregation is a process when the relation between two entity is treated as a single entity. Here the relation between Center and Course is acting as an Entity in relation with Visitor.
Reference: The information is gathered from the internet of all the web sources.

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