What is self-join and what is the requirement of self-join?

A SELF JOIN is used to join a table with itself. This join can be performed using table aliases, which allow us to avoid repeating the same table name in a single sentence. It will throw an error if we use the same table name more than once in a single query without using table aliases.
A SELF JOIN is required when we want to combine data with other data in the same table itself. It is often very useful to convert a hierarchical structure to a flat structure.