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.

A foreign key is a column or a set of columns in a table that refers to the primary key of another table. The primary key is a unique identifier for each row in a table, such as an ID number or a ...

By using an ER model, you can identify the primary keys and the foreign keys of each table, and how they relate to each other. Add your perspective Help others by sharing more (125 characters min ...

The foreign key in the database table where it resides is actually the primary key of the other table. Here’s an example to make this clearer: If we have a database table labeled, say, “orders,” and ...

The workaround suggested in the link to mysql bug report is to recreate a foreign key index in the same mysql statement right after a unique index is dropped. Unfortunately, the way the dropUnique() ...