SQL SELF JOIN

SQL SELF JOIN is a type of join operation in Structured Query Language (SQL) used to join a table with itself. A self-join is useful when we want to compare rows within a table and retrieve the data based on the relationship between the rows.

Employees Table:

ID	Name	Manager
1	William	3
2	Alice	3
3	Bob	4
4	Charlie	NULL

Suppose we want to retrieve the name of the employee and the name of the employee's manager. In that case, we can use the SELF JOIN to join the Employees table with itself as follows:

1SELECT e1.Name as Employee, e2.Name as Manager
2FROM Employees e1
3JOIN Employees e2
4ON e1.Manager = e2.ID;

The output of the above query will be:

Employee	Manager
William	Bob
Alice	Bob
Bob	Charlie

As we can see from the output, the SELF JOIN returns the name of the employee and the name of the employee's manager. The employee with ID 4 has no manager, so there's no matching row in the Employees table, and the result contains a NULL value for the Manager column.