Comment 7 for bug 1676284

I see this is expired but I am running into this scenario again and the steps to reproduce and fix this seem to be simple so I'm posting it here because backlinks lead me here. This can occur in many Ubuntu versions and it's essentially user error.

REPRODUCE:
The basic sequence and user reasoning appears to be:
1. User installs mysql-server

2. User decides to switch to mariadb, so he removes mysql packages but he doesn't purge the data directory

3. Then after installing mariadb, the service fails to start because the data directory is initialized by mysql. The person then decides to remove the mysql data manually.

4. So the user uninstalls all mariadb packages and then manually deletes the /etc/mysql and /var/lib/mysql directories.

5. But mysql-common is still lingering because it was installed as a dependency of mariadb-common. So the manual deletion of /etc/mysql left mysql-common in a broken state.

6. Now any attempts to install mariadb-server (or merely mariadb-common) will cause the installation to fail with this error:
update-alternatives: error: alternative path /etc/mysql/mariadb.cnf doesn't exist

A command sequence to reproduce this error is:
$ sudo apt -y purge 'mysql-*' 'mariadb-*' && sudo apt -y install mysql-server && sudo apt -y remove 'mysql-*' && sudo apt -y install mariadb-server && sudo apt -y remove 'mariadb-*' && sudo rm -rf /etc/mysql /var/lib/mysql && sudo apt -y install mariadb-common

CONCLUSION AND FIX:
To avoid the user error use the "apt purge" when removing mysql instead of "apt remove" with manually deletion of mysql configuration/data, because this leaves mysql-common broken.

If you've run into this problem, a simple fix is:
$ sudo apt -y purge mysql-common && sudo apt -y install mariadb-server