Core Concept Of ERD (Entity Relationship Diagram)

What is an Entity Relationship Diagram?

Entity Relationship Diagram (ERD) is a visual representation of the underlying data model in the database.

It depicts various entities along with their attributes and relationships with other entities and helps visualize the structure of database objects.

Advantages of ERD

1. It depicts the complexities of the underlying database in a visual structure. This helps in structured depiction and communication.
2. ERD helps in the design of the data model by identifying different elements and relationships among the elements.

Elements of ERD

The main building blocks of ERD are given below along with their symbols,

Entity

All nouns such as person, place, object, and event qualify as an entity in the ERD. An entity usually has attributes that define its properties.

Examples of entities include Address Details, Person Details, Vehicle details, Student details, etc.

An entity is depicted by the following symbol.

Entities are termed as "weak entities" if they depend on another entity for their existence. For instance, an entity such as "Item" which depends on another entity "ItemDetail" is a weak entity.

They are denoted by the following symbol,

Attributes describe the properties of an entity. For instance, a Student Entity has attributes such as Studentld, Studentlvame, StudentAddress, and StudentPhone.

It is denoted by the following symbol,

Key attributes are the ones that uniquely identify the entity. In the above example, StudentId is the key attribute as it can uniquely identify a student.

It is denoted by the following symbol,

Attributes can also be multi-valued if they contain multiple values such as names of countries. This is depicted by,

Relationship depicts the relationship between two entities and how the data is shared across two entities. Cardinality depicts the number of instances of one entity related to an instance of another entity. It can be one-to-one one-to-many or many-to-many.

The following diagram depicts one-to-many cardinality in the relationship as the Department entity has many students

The complete participation of entity OrderDetail in the relationship is depicted as follows,

Some entities can have "self-relationships" wherein they relate to themselves. One example is the "Employee" entity relates to itself through the "manage" - relationship as the Manager is also an employee but he/she manages at least one other employee.

Example of Development of an ERD

The following is the ERD for modeling an employee and his/her relationship with a department in an organization. After applying the above steps, we identify two main entities, Employee and Department, which are related by the "Assigned To" relationship.

All main attributes of an Employee are identified and "Employed" is the primary key attribute that uniquely identifies an employee. Similarly, all main attributes of the Department entity are identified with "Deptld" as the primary key attribute.

The example of ERD is shown in the below figure.

Summary

I hope you understood this article. We have learned the fundamentals of E-R Diagram. This article is useful to all final-year students.