For ages, I've had an issue that I could not get MySQL to start. I have MariaDB installed and it runs fine, but following the steps of
brew services stop mariadb brew unlink mariadb brew install mysql brew services start mysql
always ended up with
started in yellow for the MySQL service.
Turns out legacy data from a previous MySQL version (5.5) was the issue.
/usr/local/var/mysql there are these files:
(I don't know if there's ever more than the two.)
These are the InnoDB redo log files, and the format has changed between versions 5.x and 8. Moved these files out of the directory (kept them just in case, initially) and MySQL now runs no problem and I can switch between that at MariaDB as needed (which is a thing I actually need to do, as much as it annoys me).
Unfortunately any older projects that relied on MySQL 5.x are just plain out of luck at this point.