I updated my development computer to OS X 10.10 (Yosemite) and found MySQL was dead. These are notes on what I did to get MySQL running again.
Update the latest MySQL off of their website. At the time of this writing, that is Mac OS X 10.9 (x86, 64-bit), DMG Archive from the MySQL download site.
I found that MySQL would run but MySQL Workbench couldn’t connect. It turns out that the data directory is kept under each version. Thus, when I updated from 5.6.17 to 5.6.21 MySQL was pointing to a fresh new database.
The new version is installed in /usr/local/mysql-5.6.21-osx10.8-x86_64, with the MySQL data files in the data subdirectory. I renamed the data subdirectory to ensure I have a backup and made a copy of the old directory.
sudo cp -rp ../mysql-5.6.17-osx10.7-x86_64/data .
It appears that MySQL is working fine now. Note that this will only work for incremental revisions. If you’re reaching across minor version numbers (such as 5.5 to 5.6), the changes in structure will probably not work. (You did make a backup of the original data directory, didn’t you?)