Remember that MySQL was always dual-licensed
@JEDIDIAH; Remember that MySQL was- and is- dual licensed. Contributions to the official MySQL development stream required their copyright to be signed over to the MySQL company (later owned by Sun).
This means that they could- and did- offer it under non-GPL terms to customers who preferred that for various reasons. (That, in fact, *was* the business model, along with support, AFAICT). This doesn't negate any rights offered by its pre-existent release under GPL terms, it's simply the copyright owner exerting their right to offer it to other people under different terms if they so wish.
The same dual-licensing option is theoretically open to any GPLed project, but isn't usually practical if there are a large number of disparate copyright holders who must all agree to it; now you can see why MySQL wanted the *ownership* of the copyright signed over to them, even though they agreed to- and did- release it under the GPL.
Thus Sun retained that right when they bought MySQL, as now do Oracle. Oracle could in theory build a non-GPL version of MySQL on top of the existing version, and make all future versions non-free. They can't stop people forking the GPLed versions (even if they had *additionally* been released under an alternate license too) and they can't pull off the same trick with forks of those versions- such as MariaDB- since the copyright to subsequent changes won't have been signed over to them.
(OTOH, MariaDB can't create a non-free version of itself without either getting Oracle and all the other copyright holder's permission and/or rewriting all those parts!)