Self join

SELF JOIN : - self join is a regular join in which a table is joined to itself . self join is powerful for comparing values in a column of rows with the same table . 
Syntax : - 
SELECT  column_name(s)
From table1  AS t1
Join table1 as t2 
ON t1. Col_name = t2.Col_name;
Table1 = table name
Table Employee

emp_id       emp_name       managerid
     1         Ankit           NULL
     2         Vikas            1
    3          Monu             1
    4          Anisha           2
    5          David            2


emp_id: Unique ID for each employee.
emp_name: Name of the employee.
manager_id: The ID of the employee's manager (references emp_id). If manager_id is NULL, the employee has no manager

Find the Name of Employees Who Have a Manager
SELECT e.emp_name AS Employee, m.emp_name AS Manager
FROM Employees e
JOIN Employees m ON e.manager_id = m.emp_id;


Find All Employees, Including Those Without a Manager
SELECT e.emp_name AS Employee, m.emp_name AS Manager
FROM Employees e
LEFT JOIN Employees m ON e.manager_id = m.emp_id;



a self-join in SQL works as an inner join when no LEFT JOIN or RIGHT JOIN is specified.
If we want to include all rows, even when there is no match (e.g., employees without a manager), we need to use a LEFT JOIN.