The database folder is called storage_1 This is where all of your player and world persistence are. The db folder is for tweaking spawn rate adding more item changing the economy. ect. So if you haven't modified anything in that folder then all you have to import is the storage_1 folder.
storage_1 folder is created only once you start the server so this mean even if you update this would not be replace or erased. Everything else such db folder and other file and folder would get replaced. This is why you should always backup before upgrading.