SELECT location_id, department_name "Department", 
   TO_CHAR(NULL) "Warehouse"  FROM departments
   UNION
   SELECT location_id, TO_CHAR(NULL) "Department", warehouse_name 
   FROM warehouses;
---------------------------
SELECT location_id, department_name AS "Department", TO_CHAR(NULL) AS "Warehouse"
FROM departments
UNION
SELECT location_id, TO_CHAR(NULL) AS "Department", warehouse_name
FROM warehouses;