Is the parent_id a self reference and null?

Moving on to Bill Karwin's book SQL Antipatterns, Chapter 3, Naive Trees (Adjacency Table, Parent-Child Relationship), there is an example of a comment table.

CREATE TABLE Comments (
comment_id SERIAL PRIMARY KEY,
parent_id BIGINT UNSIGNED,
comment TEXT NOT NULL,
FOREIGN KEY (parent_id) REFERENCES Comments(comment_id)
);

      

        
        
        
      

    

Sample data

| comment_id | parent_id | comments
|------------| ----------|-------------------------------------
|1           | NULL      |What’s the cause of this bug?
|2           | 1         |I think it a null pointer
|3           | 2         |No, I checked for that
|4           | 1         |We need to check for invalid input
|5           | 4         |Yes,that a bug
|6           | 4         |Yes, please add a check
|7           | 6         |That fixed it

      

        
        
        
      

    

The table has comment_id, parent_id and a comment column. Parent_id is a foreign key referencing comment_id.

Auto add comment_id starting at 1.

Question.

If parent_id should be a foreign key that refers to comment_id, then why does the line with comment_id = 1 have parent_id null / 0 when the target is to have a foreign key to ensure referential integrity.

Note. I created the table as is and tried to enter data and got this error

# 1452 - Unable to add or update child row: foreign key constraint failed (`category`.`comments`, CONSTRAINT` comments_ibfk_1` FOREIGN KEY (` parent_id`). REFERENCES` comments` (`comment_id`))