Sorry if this is a basic question. I have a Linode VM on which I have installed Mattermost which uses MySql which I installed as part on the Mattermost install. I am intending to install Nextcloud on the same VM. Nextcloud requires MariaDB, apt-get install mariadb-server. Should there be any problem with running this install if MySql was previously installed, will MySql be reinstalled an wipeout the Mattermost database.
Thanks much,
Dennis
Do this only if you are very brave. No backup. No mercy.
Mariabd installation will replace MySQL. If I remember well I did this once by accident and it worked. Because they are more or less compatible. But there is absolutely no warranty.
It works means after the mariadb installation you’ll have again a process named mysqld running on your system. And it also will open your mm database files. Probably.
Use a second vm. Use docker container. Use postresql.
Nextcloud runs fine with a recent MySQL version (5.7 or newer), so i would just use the already running MySQL server.
I am also running a seperate VM for my MySQL server, which nit only hosts my NC database but a bunch of other as well.
The NC installer itself doesn’t wipe out any existing MariaDB installation or database. However, you should know the name of your Mattermost database and make sure, that you don’t use the same name for your NC database - although the chances are near to zero, that this will happen… 
As of to what @Reiner_Nippes said: I’d perform a full mysqldump before installing the NC instance, just to be safe.
My Mattermost installation is very recent, 2 weeks ago, so our team is not fully using it yet. So I think it would make sense to let the NC installer process install MariaDB. From what I understand MariaDB will run in place of MySQL and Mattermost should be OK. Does that make sense?
Well… I don’t know, which “Installer” you’re using, but originally, NC is only a tar.gz archive, which will unpacked and copied to your web server. So basically, NC expects MySQL/Maria DB to be already installed.
If you’re using some 3rd party installer, this may of course be different and you’d have to check, if that installer takes it into account, if there’s already a Maria DB instance installed.