While the MySQL code is GPL(v2)ed the MySQL documentation is proprietary. There may be many forks of MySQL, but I have been unable to find any with useful documentation. Who wants to use a database that has no documentation? IANAL but it seems that re-creating the reference manual for a database, what should be the specifications for the code, is more than a huge task it is also fraught with legal risk should reference to the actual MySQL docs be part of the re-creation process. (There may be legal risk in any case. If nothing else in the threat of a lawsuit.)
To my mind the proprietary nature of the MySQL docs is a large dis-incentive to fork, one strongly reminiscent of traditional proprietary vendor lock-in. It puts MySQL in an entirely different category from the usual FOSS project when it comes to forking.
I would be entirely comfortable with any sale of the MySQL codebase -- if the documentation had a free license. As things stand the owner of the MySQL documentation has control over MySQL in a manner that's not comparable to other projects -- there are no forks of a similarly licensed code/documentation pair of which I am aware. This makes me very skeptical of arguments claiming that MySQL can be forked. I have seen no reasonable arguments that consider what it would take to fork the documentation along with the code.