I think a natural key that I’ve encountered is a database containing professional NBA data. Teams never play more than once per day, and there are only two team

I think a natural key that I’ve encountered is a database containing professional NBA data. Teams never play more than once per day, and there are only two teams in the game, so a natural (composite) key for games is simply the date of the game and the two teams that played. You can create a “game ID” if you wish, but you aren’t actually doing anything but adding a column of redundant information.