Is it possible to transfer just an app's mysql/mariadb database info into a new installation of nextcloud?

[Bernie_O]

@averageadmin

For example, I’d like to transfer over only the Calendar and Deck apps’ data to a fresh installation. I think that in theory this should be possible, but I’d like to know how many different tables this may involve for each app. If each app just uses one table, that should be pretty easy to copy into a new mariadb database.

I can’t say anything about the Deck app, but the relevant data for calendars is stored in these tables - some of those might have been introduced after Nextcloud 19 (your table prefix might be different from oc_):

oc_calendarchanges
oc_calendarobjects
oc_calendarobjects_props
oc_calendars
oc_calendarsubscriptions
oc_calendar_invitations
oc_calendar_reminders
oc_calendar_resources
oc_calendar_resources_md
oc_calendar_rooms
oc_calendar_rooms_md
oc_dav_cal_proxy
oc_dav_shares
oc_group_user

As calendardata is spread over several tables and also synchronization status and other information is stored along with the data, I think just directly importing these tables is quite error-prone.

If there are not to many users and calendars and as long as you have access to the database of the broken instance, you could also export the calendardata to *.ics files with calcardbackup and then import the *.ics files to the new instance.