uuid

The uuid should have no collision. uuid1 seems to be the better choice, since there's almost no chance that someone would spoof their own mac address. uuid4 has the disadvantage of having a small collision chance, which isn't optimal in a database.

http://stackoverflow.com/questions/703035/when-are-you-truly-forced-to-use-uuid-as-part-of-the-design/786541#786541