Self Joins

A self join joins a table to itself. You use aliases to treat the same table as two separate tables.

This is useful for hierarchical data like employee/manager relationships:

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

Key points:

• You MUST use aliases to distinguish the two copies
• LEFT JOIN ensures employees without a match still appear
• Common for org charts, referral chains, and category trees

Self joins are a powerful pattern once you understand them.