Show the ids of all employees who have either destroyed a document or made an authorization to do this.
create table documents_to_be_destroyed (destroyed_by_employee_id varchar, destruction_authorised_by_employee_id varchar, PRIMARY KEY (destroyed_by_employee_id))
select destroyed_by_employee_id from documents_to_be_destroyed union select destruction_authorised_by_employee_id from documents_to_be_destroyed