Key and Superkey:

Example:

Let R be a relation schema with schema(R) = {A₁, A₂, B₁, B₂, C}.
Let F = { A₁ -> B₁, A₂ -> B₂, B₁ -> A₁, B₂ -> A₂, B₁B₂ -> C}.

Keys for R with respect to F:

A₁A₂C, B₁B₂C, A₁B₂C, B₁A₂C.

Any set of attributes Y with a subset X which is key for R is a superkey.
Try to list all superkeys for R with respect to F.

(Keys for relations in DD Airline Scheduling will be shown in each kind of normal form respectively.
For detail, go to Redundancy Reduction).