Store different rows type in single table

You need to have already declared the table that the foreign key references, before you can define a foreign key that references it.

Once you declare the second table, you can then declare the first table. Tested here on MySQL 5.5.27.

If you need to bypass the validation, you can do this:

SET FOREIGN_KEY_CHECKS = 0;
-- declare tables
SET FOREIGN_KEY_CHECKS = 1;

http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_foreign_key_checks

Solution 1:

Make the event_id nullable.

ALTER TABLE events
  MODIFY COLUMN event_id int unsigned NULL ;

This will allow you to insert rows which reference nothing (NULL):

INSERT INTO events
  (user_id, event_id, title)
VALUES
  (a_valid_user_id, NULL, 'test event 1') ;

And to see events without a "parent" event:

SELECT *
FROM events
WHERE event_id IS NULL ;

Test at SQL-Fiddle-1

(Parenthesis)

Unless you have some braindead ORM that requires all tables to have a column named id, it's better to have names that describe what they represent. I'd rename id to event_id and event_id to parent_event_id:

CREATE TABLE events (
  event_id int(10) unsigned NOT NULL AUTO_INCREMENT,
  user_id int(10) unsigned NOT NULL,
  parent_event_id int(10) unsigned NULL,
  title varchar(200) NOT NULL,
  PRIMARY KEY (event_id),
  --- ...
) ;