2

I got a little confused by all these keys. As I understand

A key of a relational schema = 
The minimal subset of a superkey that is still a key = 
A candidate key

Is this correct or not?

4

Just found out very neat explanation:

superkey: a set of attributes which will uniquely identify each tuple in a relation

candidate key: a minimal superkey

primary key: a chosen candidate key

secondary key: all the rest of candiate keys

prime attribute: an attribute that is a part of a candidate key (key column)

nonprime attribute: a nonkey column

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