Well, that’s because we sometimes need different ways to describe what we’ve got going on. Of the four different types of keys we’ve discussed so far, they are all different enough that we need to differentiate them and be able to explain what the differences are. For the rest of the keys that we’ll go through today, the same idea exists. They are close to the others but different enough that there is a need for another name for that type of key.
There is a super key.
Read on to learn what a superkey is. That will put you one quarter of the way to understanding Boyce-Codd Normal Form: a relational variable is in Boyce-Codd Normal Form if and only if all functional dependencies have superkey determinants.