text: -- examples on aggregate functions
  use hron;
  
  -- count, count distinct
  select count(*), count(manager_id), count(distinct manager_id), count(commission)
  from employee;
  
  -- max, min
  select max(salary), min(salary)
  from employee;
  
  -- min, max on dates
  select min(hired), max(hired)
  from employee;
  
  -- sum
  select sum(salary)
  from employee
  where department_id = 5;
  
  -- average
  select round(avg(salary), 2)
  from employee
  where department_id = 5;
  