what is use of self join and which cases you will use this join

what is use of self join and which cases you will use this join..

Answer / vidhya

A physical file can be joined to itself to read records that are formed by combining two or more records from the PF itself.
For example
You use a self join when a table references data in itself.

E.g., an Employee table may have a SupervisorID column that points to the employee that is the boss of the current employee.

To query the data and get information for both people in one row, you could self join like this:

select e1.EmployeeID,
e2.FirstName as SupervisorFirstName,
e2.LastName as SupervisorLastName
from Employee e1
left outer join Employee e2 on e1.SupervisorID = e2.EmployeeID

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