It depends exactly how you deploy your software. If you distribute your code on its own, then include instructions as to how the end user must configure MySQL to enable the 2 to work together correctly this is ok (like vBulletin for example). You can't include MySQL in a distributable package that has to be paid for. If this is what you want, you must purchase a liscense.
Basically, if your package is 100% GPL you can use MySQL for free, otherwise you have to pay.
MySQL Licensing
Selling products that use MySQL
Regards,
Matt.
