i didnt understand what is the self join and what does it do exactly ?

Hi Delih!
Thanks for reaching out.

A self join is a regular join that is used to join a table to itself. It's useful when you want to compare rows within the same table. To perform a self join, you treat the same table as two separate ones by giving them different aliases (i.e., different names). 

For example, imagine you have a table of employees where each employee has an ID and a manager ID, and you want to list all employees along with their managers. Since both employees and managers are in the same table, you would self join the table on the employee's manager ID to the manager's employee ID.

Here's a simple SQL example:

SELECT e1.name AS EmployeeName, e2.name AS ManagerName
FROM employees AS e1
JOIN employees AS e2 ON e1.manager_id = e2.employee_id;

In this example, `e1` and `e2` are aliases for the `employees` table, used to differentiate the employee from the manager in the self join. The query joins two instances of the `employees` table based on the relationship where one employee's `manager_id` column matches another employee's `employee_id` column.

Hope this helps.
Best,
Tsvetelin