Now we are entering one of the most important concepts in SQL:

JOIN

This is where SQL starts to feel like real backend development.

The Problem We Want to Solve

We want to answer this question:

For each comment, show:

• The content of the comment
• The username of the person who wrote the comment

Example output:

contents

username

This photo is amazing

Alfred66

Wow!

Reyna

Nice work

Micah

Notice something important:

• contents comes from comments table
• username comes from users table

That means:

👉 We need data from two different tables.

And that is our first hint that we need a JOIN.

Step 1: Understand the Relationship

Let’s look at the structure:

comments table

• id
• contents
• user_id (foreign key)
• photo_id

users table

• id
• username

The key connection:

comments.user_id → users.id

This foreign key tells us:

Which user wrote each comment.

Step 2: Write the Query

Here is the SQL:

SELECT contents, username
FROM comments
JOIN users
  ON users.id = comments.user_id;

That’s it.

Now let’s understand what this actually does.

What Happens Behind the Scenes?

1️⃣ FROM comments