Would you please show me the ids of all employees who do destroy any documents?
SELECT employee_id FROM Employees as t1 join Documents_to_be_destroyed as t2 on t1.Employee_ID  =  t2.Destroyed_by_Employee_ID
ids of all employees who do destroy any documents refers to joining Employees table with Documents_to_be_destroyed table on employee_id = Destroyed_by_Employee_ID;