Summary: On Ubuntu 20.04 Server Edition, MariaDB 10.3, once installed while MySQL server is running, fails to launch after installation.

Steps to Reproduce:

1. Install Ubuntu 20.04 Server.
1. Install MySQL server on an Ubuntu 20.04. (apt install mysql-server)
1. Reboot.
1. While the MySQL server is running, install MariaDB 10.3. (apt install mariadb-server-10.3)

Expected Result: MariaDB 10.3 should launch successfully after installation.

Actual Result: MariaDB 10.3 fails to launch after installation.

```
sudo systemctl status mysql
[sudo] password for ubuntu:
● mariadb.service - MariaDB 10.3.38 database server
Loaded: loaded (/lib/systemd/system/mariadb.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Mon 2023-07-17 05:11:12 UTC; 32min ago
Docs: man:mysqld(8)
https://mariadb.com/kb/en/library/systemd/
Main PID: 2503 (code=exited, status=1/FAILURE)

Jul 17 05:11:11 ubuntu systemd[1]: Starting MariaDB 10.3.38 database server...
Jul 17 05:11:12 ubuntu systemd[1]: mariadb.service: Main process exited, code=exited, status=1/FAILURE
Jul 17 05:11:12 ubuntu systemd[1]: mariadb.service: Failed with result 'exit-code'.
Jul 17 05:11:12 ubuntu systemd[1]: Failed to start MariaDB 10.3.38 database server.
```

Additional Information: When following the same steps on Ubuntu 22.04 LTS, no such error occurred and MariaDB (10.6) launched as expected.

Additional Comments: While I understand this may not be a common or recommended operational scenario, I believe it's essential for software to handle such situations gracefully. Ideally, one should create a dump using mysqldump before making such changes, and I believe recovery would be possible in such a case. However, if a dump was not created, there is a potential risk of data loss. Unexpected behavior like this could lead to confusion and could potentially disrupt services.

—
Please note that I am a native Japanese speaker and used a translation app to write this report.