One-to-One and Many-to-Many Relationships (With Real-World Examples)

So far, we’ve already discussed one-to-many and many-to-one relationships, which are the most common relationships you’ll use in real applications.

In this lesson, we’re going to look at two more important relationship types:

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

⚠️ Important note before we begin:

These two relationships do not exist in the photo-sharing database we are currently building.

We’re learning them now so you’re aware they exist and can recognize them later.

You will see real implementations of these relationships later in the course.

Why Learn All Relationship Types?

In databases, relationships define how data connects.

There are four major relationship types you should know:

1. One-to-One 2. One-to-Many 3. Many-to-One 4. Many-to-Many

You already understand One-to-Many / Many-to-One.

Now let’s complete the picture.

One-to-One (1:1) Relationship

What is a One-to-One Relationship?

A one-to-one relationship means:

One record in a table is related to exactly one record in another table

— and vice versa.

That sentence sounds confusing at first, so let’s immediately look at examples.

Example 1: Boat and Captain

• A boat has one captain
• A captain is assigned to one boat

Boat ↔ Captain

Yes, in the real world exceptions may exist, but :