find the name of all departments that do actually have one or more employees assigned to them.
SELECT DISTINCT T2.department_name FROM employees AS T1 JOIN departments AS T2 ON T1.department_id  =  T2.department_id
departments with one or more employees assigned refers to performing an INNER JOIN on employees and departments tables where employees.department_id matches departments.department_id; DISTINCT is used to ensure each department name is listed once.