Types of Relationships

Back to your café  again…

You notice something interesting:

• One customer can place many orders
• One order belongs to only one customer
• Some orders contain multiple items, and one item can appear in many orders

So not all connections are the same
Some are one-to-one, some one-to-many, and some many-to-many.

That’s exactly what Types of Relationships in the ER Model explain.

Relationships in ER models are classified based on how many entities are connected.

There are three main types:

1. One-to-One (1:1)
2. One-to-Many (1:N)
3. Many-to-Many (M:N)

These define how data interacts inside a system.

One-to-One (1:1) Relationship

In this type, one entity is related to only one other entity.

Example:

• One person has one passport
• One passport belongs to one person

Code Example

Technical Example

Real Life Example

• Person → Has → Passport
• User → Has → Unique Profile

One-to-Many (1:N) Relationship

In this type, one entity can be related to many entities,
but each of those belongs to only one.

Example:

• One customer places many orders
• One order belongs to one customer

Technical Example

Real Life Example

1. College System
   • One teacher teaches many students
2. E-commerce
   • One user places many orders
3. Bank System
   • One customer has multiple accounts

Many-to-Many (M:N) Relationship

In this type, many entities are related to many others.

Example:

• Students enroll in multiple courses
• Each course has multiple students

Technical Example

Here, Enrollment is a bridge (junction table)

Real Life Example

1. College System
   • Students ↔ Courses
2. Online Shopping
   • Orders ↔ Products
3. Social Media
   • Users ↔ Friends

Understanding relationships is the heart of ER modeling:

1:1 → One to one
1:N → One to many (most common)
M:N → Many to many (requires bridge table)

If you get relationships right, your database will behave correctly.