I need help with relationships between parent category and child category on a innodb engine.

[eluser]weboide[/eluser]
I believe that for a foreign key to be optional, it has to be possible to set it to NULL (ie without NOT NULL).

Try with parent_id like this:

Code:

`parent_id` int(10) unsigned DEFAULT NULL

If there is no parent, just set parent_id to NULL or do not set it (as it is by default set to NULL)