select distinct e.first_name, e.last_name from employees e join employees m on e.employee_id= m.manager_id;

select distinct department_name, first_name, last_name, hire_date, salary, (sysdate-hire_date)/365 as experience,  e.manager_id, e.employee_id from employees e join departments d on e.employee_id = d.manager_id where (sysdate-hire_date)/365 > 15;

select first_name,last_name, salary from employees where salary > (select avg(salary) as salary from (select avg(salary)as salary from employees group by department_id));

select max(salary) as highest_salary, min(salary) as lowest_salary, sum(salary) as sum_, avg(salary) as avg_salary from employees;