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

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i'm creating tree view website using databases. create 2 tables, let's call them "folder" , "file". folder can contain multiple folders , files, , files act leaf nodes - means they're "dead end" in tree if you're unfamiliar term.

the folder table contain columns: 

folder_id, folder_name, folder_parent

file contain: 

file_id, file_name, file_parent

obviously file_parent reference folder_id, creating foreign key, how make folder_parent reference folder_id?

example:

folder_name   | folder_id   | folder_parent root               1               null  cars               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) )

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