As you may know, MySQL has a dual-licensing model. You can get the source under the GPL version 2, or you can buy a commercial license.
I’ve recently been hearing a lot of confusion about when you have to buy a commercial license. People I’ve spoken to wrongly believe that they’re required to purchase a license if they’re going to use MySQL in anything but a not-for-profit business, for example. I don’t know how these notions get started, but they do.

Mas:

http://www.xaprb.com/blog/2009/02/17/when-are-you-required-to-have-a-commercial-mysql-license/