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Let be the following database sketch:

Cineroom(Name, Hour, Name)

Film(Name, Director, Actor)

Saw(Spectator, Name)

Produce(Producer, Name)

Likes(Amateur, Name)

How to express, in relational algebra the difference between:

  • which actors played in a film they produced?
  • which actor produced a film?

To my mind it was the difference between Cartesian product and outer join:

ΠActorProducer=Actor(Film ⋈ Produce))

ΠActorProducer=Actor(Film*Produce))

Would it allow to join only the films the actor played and produced in the first case?

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