SELECT employee_id, emp_name, dept_name, salary,
       LAG(salary, 1, 0) OVER (PARTITION BY dept_name
                                   ORDER BY salary DESC
                              ) lag_previous,
       LEAD(salary, 1, 0) OVER (PARTITION BY dept_name
                                    ORDER BY salary DESC
                                ) lead_next
  FROM emp_hierarchy
 ORDER BY 3, 4 DESC;