Foreign Keys: The Glue Between Tables

What is a Foreign Key?

A foreign key is a column that references the primary key of another table. It creates a relationship between tables.

Example

-- sessions.user_id → users.id
-- This means every session belongs to a user

sessions	users
session_id (PK)	id (PK)
user_id (FK)	→	id
started_at	username

Why Foreign Keys Matter

• Data integrity: Can't create a session for a non-existent user
• Query relationships: JOIN tables on foreign keys
• Performance: Foreign keys are often indexed automatically

Your Task

Use the foreign key relationship to find which users have sessions:

• Join users and sessions on the foreign key
• Show: user id, username, session count
• Only include users with at least 1 session
• Order by session count descending