text: -- examples on null
  alter session set current_schema = hron;
  
  -- check null
  select employee_id, first_name, last_name, commission
  from employee
  where last_name = 'King' and commission is null;
  
  -- !!! WRONG - no row is returned !!!
  select employee_id, first_name, last_name, commission
  from employee
  where last_name = 'King' and commission = null; -- !!! WRONG !!!
  
  -- check not null
  select employee_id, first_name, last_name, commission
  from employee
  where last_name = 'King' and commission is not null;
  
  -- !!! WRONG  - no row is returned !!!
  select employee_id, first_name, last_name, commission
  from employee
  where last_name = 'King' and commission != null;
  
  -- null in operations, null is returned
  select first_name, last_name, salary * commission
  from employee
  where last_name = 'King';
  
  -- using nvl() to get rid of null
  select employee_id, first_name, last_name, salary * nvl(commission, 0) as "Monthly Commission"
  from employee
  where last_name = 'King';
  
  -- using nvl2()
  select first_name, last_name, nvl2(commission, 'value', 'no value') as commission
  from employee
  where last_name = 'King';
  
  -- coalesce()
  select coalesce(null, null, 42, null)
  from dual;
  