MySQL : ERROR 1215 (HY000): Cannot add foreign key constraint

Question

I have read Database system concepts, 6th edition, Silberschatz. I\'m going to implement the university database system shown in chapter 2 on OS X

Answer 1

It is also possible to get this error if the foreign key is not a primary key within its own table.

I did an ALTER TABLE and accidentally removed the primary key status of a column, and got this error.