My mysql database (5.6.23) became unable to start when I upgraded from El Capitan to Catalina (10.15.5). Now it works. This is what I did, but its possible some of this might not be needed or a recommended thing to do:
/tmp# cd /usr/local/mysql/data
/usr/local/mysql/data# chown _mysql:_mysql *
/usr/local/mysql/data# chmod 777 *
After hours investigating I found this post from Michael Fraase: https://discourse.brew.sh/t/mysql-8-0-18-on-catalina-wont-run-as-service/6015/2
I just added:
to /etc/my.cnf (or where you have your .cnf file) and it solved it (I can not believe it).