I had to remove and reinstall MariaDB to upgrade from Fedora 29. I assumed Fedora 31 would have a more recent version, so I just installed the database and attempted to start it. Nope.

In journalctl

Dec 28 23:37:49 fedora123 mysql-prepare-db-dir[3977]: Database MariaDB is probably initialized in /var/lib/mysql already, nothing is done.
Dec 28 23:37:49 fedora123 mysql-prepare-db-dir[3977]: If this is not the case, make sure the /var/lib/mysql is empty before running mysql-prepare-db-dir.
Dec 28 23:37:49 fedora123 mysqld[4013]: 2019-12-28 23:37:49 0 [Note] /usr/libexec/mysqld (mysqld 10.3.20-MariaDB) starting as process 4013 …
Dec 28 23:37:49 fedora123 systemd[1]: mariadb.service: Main process exited, code=exited, status=1/FAILURE

In mariadb.log

2019-12-28 23:23:53 0 [ERROR] Fatal error: Can’t open and lock privilege tables: ‘mysql.user’ is not of type ‘TABLE’

Turns out the Fedora repo has 10.3, but the MariaDB repo that I’d used with Fedora 29 had MariaDB 10.4. After enabling the MariaDB repo and installing 10.4, the service without error.