What is a self-join?

A self-join means joining a table to itself.

Example: employees and their managers are stored in the same table.

employees
---------
id
name
manager_id

Query:

SELECT e.name AS employee,
       m.name AS manager
FROM employees e
LEFT JOIN employees m
    ON e.manager_id = m.id;

We use aliases because the same table appears twice.

Common use cases:

• Employee → manager relationships
• Hierarchies
• Finding related records
• Comparing rows within the same table