There are so many problems with foreign key constraints that we don't know where to start: Foreign key constraints make life very complicated, because the foreign key definitions must be stored in a ...

Indexes in MySQL are used to speed up access in the database and help enforce database constraints (such as UNIQUE and FOREIGN KEY). Database indexes are much like book indexes.

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.