Error added: 2013-07-30T12:07:44Z
http://dev.mysql.com/doc/refman/5.6/en/create-table-foreign-keys.html Before MySQL 5.6.7, using ALTER TABLE to change the definition of a foreign key column could cause a loss of referential integrity. For example, changing a foreign key column that contained NULL values to be NOT NULL caused the NULL values to be the empty string. Similarly, an ALTER TABLE IGNORE that removed rows in a parent table could break referential integrity. As of 5.6.7, the server prohibits changes to foreign key columns with the potential to cause loss of referential integrity. A workaround is to use ALTER TABLE ... DROP FOREIGN KEY before changing the column definition and ALTER TABLE ... ADD FOREIGN KEY afterward.
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