-- UNION
select emp_id as person_id
from employee
union
select cust_id as person_id
from customer;

-- UNION ALL
select emp_id as person_id
from employee
union all
select cust_id as person_id
from customer;

-- INTERSECT
select emp_id as person_id
from employee
intersect
select cust_id as person_id
from customer;

-- EXCEPT
select emp_id as person_id
from employee
except
select cust_id as person_id
from customer;