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

When you get this vague error message, you can find out the more specific error by running

SHOW ENGINE INNODB STATUS;

The most common reasons are that when creating a foreign key, both the referenced field and the foreign key field need to match:

• Engine should be the same e.g. InnoDB
• Datatype should be the same, and with same length.
  e.g. VARCHAR(20) or INT(10) UNSIGNED
• Collation should be the same. e.g. utf8
• Unique - Foreign key should refer to field that is unique (usually private) in the reference table.

Another cause of this error is:
You have defined a SET NULL condition though some of the columns are defined as NOT NULL.