Constrain a columns value to a value of a different column inside the same table?

I'm creating a tree view for a website using databases. I will create 2 tables, let's call them "Folder" and "File". A folder can contain multiple folders and files, and files will act as leaf nodes - which means they're a "dead end" in the tree if you're unfamiliar with the term.

The folder table will contain columns:

Folder_id, Folder_name, Folder_parent

File will contain:

File_id, File_name, File_parent

Obviously File_parent will reference a folder_id, thus creating a foreign key, but how do I make the Folder_parent reference the Folder_id?

Example:

FOLDER_NAME   | FOLDER_ID   | FOLDER_PARENT
root               1               null 
Cars             开发者_运维问答  2                 1 
Planes             3                 1 
BMW                4                 2 

---

create table folder (
  folder_id int primary key,
  folder_name varchar not null,
  folder_parent int references folder(folder_id)
)

---

I would make a separate table to deal with relationships, since a file can easily exist in two folders (think aliases). In this schema, you could simplify things with just a node table and a relationships (or edges, if you're familiar with graph theory) table:

CREATE TABLE nodes (
    node_id int primary key,
    node_name varchar not null,
    node_type enum('folder','file')
)

CREATE TABLE edges (
    child_node_id int primary key,
    parent_node_id int,
    unique( child_node_id, parent_node_id)
)

I'm a little rusty on my sql, so my syntax may need some work, but that's how I would approach it. It's much more flexible this way.