Can you do a foreign key within a single table, though? I've only ever seen foreign key constraints tying a field in one table to a field in another table.

Hence, the birth of the MySQL InnoDB vs. MyISAM debate. Of course, MySQL users prior to the 5.5 release were probably perplexed by the assertion that the database didn't support foreign keys. MySQL ...

I wouldn't drop and add a foreign key just to modify data. In Oracle and MySQL you can temporarily disable foreign key checks.