What is the meaning of “minimal key”?

Question

In the definition, it says minimal key is a minimal set of attributes needed to identify the table. Does the "minimal" part mean "minimal number of attributes" or "cannot be eliminated"?

For example:

• For a relation R(A, B, C, D, E, F), the closure of A, BC, and DEF are all {A, B, C, D, E, F}.

Are they all minimal keys? Or is only A the minimal key?

Answer 1

They are all minimal keys.

Minimal means there is no proper subset with the same property.