I expirienced an issue with one of my CalDAV clients, which deleted some events from my calendar. So I restored the data of that one calendar from the latest backup before the malicious client got connected.

To achieve this only seven little steps are necessary:

1. Remove unrelated calendars from the backup

    DELETE FROM oc_calendars WHERE id NOT IN (1, 5, 3, 12, 22, 33, 34);
    DELETE FROM oc_dav_shares WHERE type <> 'calendar' OR resourceid NOT IN (1, 3, 5, 12, 22, 33, 34);
    DELETE FROM oc_calendarobjects WHERE calendarid NOT IN (1, 3, 5, 12, 22, 33, 34);
    DELETE FROM oc_calendarchanges WHERE calendarid NOT IN (1, 3, 5, 12, 22, 33, 34);
   

2. Save the rescued calendar alone

   Now backup (data only) the content of the 4 tables as rescue.sql.

3. Backup the calendars live data

    CREATE TABLE backup_calendars AS SELECT * FROM oc_calendars;
   

4. Delete the affected calendars on the live database

    DELETE FROM oc_calendars WHERE id IN (1, 3, 5, 12, 22, 33, 34);
    DELETE FROM oc_dav_shares WHERE type = 'calendar' AND resourceid IN (1, 3, 5, 12, 22, 33, 34);
    DELETE FROM oc_calendarobjects WHERE calendarid IN (1, 3, 5, 12, 22, 33, 34);
    DELETE FROM oc_calendarchanges WHERE calendarid IN (1, 3, 5, 12, 22, 33, 34);
   

5. Restore the rescue on the live database

    mysql -u root -p nextcloud < rescue.sql
   

6. Reset the sync token to a higher value than before

    UPDATE oc_calendars l
    JOIN backup_calendars b ON (l.id = b.id)
    SET l.synctoken = b.synctoken + 1
    WHERE l.id IN (1, 3, 5, 12, 22, 33, 34);
   

7. Delete the backup table

    DROP TABLE backup_calendars;