The staff chart loses its top row

Nothing says the two sides of a join must be different tables. The staff table has a supervisor_id that points at another row of staff. Give the table two aliases and it becomes two copies: e for the employee, m for the manager that e.supervisor_id points at. The aliases name roles, not two tables stored twice.

The bridge is ON m.id = e.supervisor_id. Write ON m.id = e.id instead and every person becomes their own supervisor, fourteen rows of nonsense that run without a complaint.

SELECT e.id AS employee_id, e.first_name AS employee, m.first_name AS supervisor
FROM staff e
LEFT JOIN staff m ON m.id = e.supervisor_id
ORDER BY e.id;

One row of staff has a NULL supervisor_id: the director, at the top of the chart. An INNER JOIN drops her, because NULL matches nothing. If the chart must show every employee, the employee copy goes on the left of a LEFT JOIN and the director keeps her row with a NULL where a supervisor's name would be.