Notes: Foreign keys in MySQL automatically create an index on the table. In this example index fk_books_author_id will be created at column books.author_id.; Setting FOREIGN_KEY_CHECKS disables ...

-- The ALTER TABLE statement is used to add, delete, or modify columns in an existing table. -- The ALTER TABLE statement is also used to add and drop various constraints on an existing table. -- ...

Foreign keys can refer to another column in the same table. The top-level folder might need to be its own parent - I don't know if it'll let you have a NULL there.