Is it possible to reference a different column in the same table?

If a blog has a 'categories' table such as the following:

CREATE TABLE categories
(
  id INTEGER PRIMARY KEY AUTO_INCREMENT,
  parent_id INTEGER NOT NULL,
  name VARCHAR(30) NOT NULL,
  description TEXT,
  count INTEGER NOT NULL DEFAULT 0
);

And if the parent_id field is intended to refer to the 'id' field of the categories table, then how could I add a constraint that would ensure that values inserted into parent_id references the id field?

I simply want to make sure that only category id values that exist can be used as a parent of a newly inserted category.