Self Join: 
is when you join a table to itself 

For example: 
if you want to display group of employees and their manager 

Employees          Manager
Neena              Steven 
lex                Steven 


//Managers and employees are all displayed together in the same table 

//a condition should be satisfied 
    Employee_id = Manager_id 

//Traditional 
select emp.first_name "Employee Name", Mng.first_name "Manager Name"
from employees emp, employees Mng
where emp.manager_id = Mng.employee_id


//ANSI Syntax
select emp.first_name "Employee name", Mng.First_name "Manager name"
from employees emp join employees Mng  
on emp.manager_id = mng.employee_id

Rules: 
- self join can be performed with attributes that are not primary key
or foregin key
- you cant use the USING clause while using the ANSI synatx, because in
this case you're using two different attributes, so it's better to use 
the ON clause 

One more example: 
Muliple join between different tables 

employees => departments => locations => countries 

select emp.first_name, dept.department_name, L.city, C.country_name, R.region_name
from employees emp join departments dept
using (department_id)
join locations L 
using (location_id)
join countries C 
using (country_id)
join regions R 
using (region_id)
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