Entity Relationship Model

Entity Relationship Model

The ER or (Entity Relational Model) is a high-level conceptual data model diagram. Entity-Relation model is based on the notion of real-world entities and the relationship between them.

ER modeling helps you to analyze data requirements systematically to produce a well-designed database. It is considered a best practice to complete ER modeling before implementing your database.

An Entity–relationship model (ER model) describes the structure of a database with the help of a diagram, which is known as Entity Relationship Diagram (ER Diagram).  An ER diagram shows the relationship among entity sets. An entity set is a group of similar entities and these entities can have attributes. In terms of DBMS, an entity is a table or attribute of a table in database, so by showing relationship among tables and their attributes, ER diagram shows the complete logical structure of a database. 

ER- Diagram Notations

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

• Rectangles: This symbol represent entity types
• Ellipses : Symbol represent attributes
• Diamonds: This symbol represents relationship types
• Lines: It links attributes to entity types and entity types with other relationship types
• Primary key: attributes are underlined
• Double Ellipses: Represent multi-valued attributes

Example of E-R diagram

Components of the ER Diagram

Entity Relationship model is based on three basic concepts:

• Entities
• Attributes
• Relationships

Entity:

An entity refers to any object having-

• Either a physical existence such as a particular person, office, house or car.
• Or a conceptual existence such as a school, a university, a company or a job.

An entity can be place, person, object, event or a concept, which stores data in the database. The characteristics of entities are must have an attribute, and a unique key. Every entity is made up of some 'attributes' which represent that entity.

Entity set:

An entity set is a group of similar kind of entities. It may contain entities with attribute sharing similar values. Entities are represented by their properties, which also called attributes. All attributes have their separate values. For example, a student entity may have a name, age, class, as attributes.

Relationship: 

Relationship is nothing but an association among two or more entities. Entities take part in relationships. 

For example, if a Student takes admission into a Class. Here Student and Class are entities. they have a association of taking admission.