I'm trying to write the relational algebra for a problem related to this schema:
I want to do the following: For each flight list the departure airport code for the first leg of the flight and the arrival airport code for the third leg.
I know how to do this for the individual parts, but I'm not sure how to select from multiple different instances of
This is what I have to start:
π airport.airport_code (flight) ⋈ flight.flight_number = leg_instance.flight_number (leg_instance) σ leg_instance.leg_number = 1