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)
[SQL] SELECT Destroyed_by_Employee_ID FROM Documents_to_be_destroyed UNION SELECT Destruction_Authorised_by_Employee_ID FROM Documents_to_be_destroyed [/SQL]