SELECT *
FROM (SELECT first_name||' '||last_name  AS "MANAGER WITH NO MANAGER" FROM employees WHERE manager_id IS NULL),
(SELECT  DISTINCT e.first_name||' '||e.last_name AS "MANAGER WITH MANAGER" FROM employees e, employees m WHERE m.manager_id=e.employee_id AND e.manager_id<>0),
(SELECT  DISTINCT m.first_name||' '||m.last_name AS "EMPLOYEE WITH MANAGER" FROM employees m, employees e
minus
SELECT  DISTINCT e.first_name||' '||e.last_name  AS "EMPLOYEE WITH MANAGER" FROM employees e, employees m WHERE m.manager_id=e.employee_id AND e.manager_id<>0),
(SELECT DISTINCT first_name||' '||last_name  AS "EMPLOYEE WITH NO MANAGER" FROM employees WHERE manager_id IS NULL AND manager_id<>employee_id)
WHERE
ORDER BY 1,2,3,4;