What is the point of a self-ref table?

Databases
1

Why would someone create an id that has a reference in the same table to some other id ? What’s the point of self referencing tables ?

Example:

CREATE TABLE Student (
id INT AUTO_INCREMENT NOT NULL,
mentor_id INT DEFAULT NULL,
PRIMARY KEY(id)
) ENGINE = InnoDB;
ALTER TABLE Student ADD FOREIGN KEY (mentor_id) REFERENCES Student(id);

That table is pulled from doctrine project reference manual in section 5.8

2

the “self reference” implements a hierarchy

here’s another example: Categories and Subcategories

3

Thanks a lot that answered some questions:)