Relationship model in DBMS

The diamond shape showcases a relationship in the ER diagram.

It depicts the relationship between two entities.

In the example below, the student and the course are entities, and study is the relationship between them. 

One-to-One Relationship

When an entity's single element is associated with another entity's single element, it is called a one-to-one relationship.

For example, a student has only one identification card, and an identification card is given to one person.

One-to-Many Relationship

When a single element of an entity is associated with more than one element of another entity, it is called a one-to-many relationship

For example, a customer can place many orders, but an order cannot be placed by many customers.

Many-to-One Relationship

When more than one element of an entity is related to a single element of another entity, then it is called a many-to-one relationship.

For example, students have to opt for a single course, but a course can have many students.

Many-to-Many Relationship

When more than one element of an entity is associated with more than one element of another entity, this is called a many-to-many relationship.

For example, you can assign an employee to many projects and a project can have many employees.

How to Draw an ER Diagram?

Below are some important points to draw ER diagram:

• First, identify all the Entities. Embed all the entities in a rectangle and label them properly.

• Identify relationships between entities and connect them using a diamond in the middle, illustrating the relationship. Do not connect relationships with each other.

• Connect attributes for entities and label them properly.

• Eradicate any redundant entities or relationships.

• Make sure your ER Diagram supports all the data provided to design the database.

• Effectively use colors to highlight key areas in your diagrams.