How to get the employees with their managers

Self -Join

SELECT e.last_name, e.employee_id, m.last_name as manager, e.manager_id, m.last_name,m.employee_id

FROM

    hr.employees  e, hr.employees  m

WHERE e.manager_id = m.employee_id

order by m.employee_id DESC

And if you want to include the president which has no manager then instead of an inner join use an outer join in Oracle syntax:

 inner join 

SELECT e.last_name, e.employee_id, NVL(m.last_name,'No Manager') as manager, e.manager_id, m.last_name,m.employee_id

FROM

    hr.employees  e, hr.employees  m

WHERE e.manager_id = m.employee_id (+)

order by m.employee_id DESC 

ANSI SQL 

SELECT e.last_name, e.employee_id, m.last_name as manager, e.manager_id, m.last_name,m.employee_id

FROM

    hr.employees  e LEFT OUTER JOIN hr.employees  m

on  e.manager_id = m.employee_id

order by m.employee_id DESC

Count All Employees Under Each Manager

SELECT 

sup.employee_id,

    sup.first_name,

    sup.last_name,

    COUNT (sub.employee_id) AS number_of_employees

FROM hr.employees sub 

JOIN hr.employees sup 

ON sub.manager_id = sup.employee_id

GROUP BY sup.employee_id, sup.first_name, sup.last_name; 

Find All Direct Subordinates Under Each Manager

SELECT 

sub.employee_id AS subordinate_id,

    sub.first_name AS subordinate_first_name,

    sub.last_name AS subordinate_last_name,

    sup.employee_id AS superior_id,

    sup.first_name AS superior_first_name,

    sup.last_name AS superior_last_name

FROM hr.employees sub 

JOIN hr.employees sup 

ON sub.manager_id = sup.employee_id

ORDER BY superior_id;