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Why does this fields have a transitive functional dependency.?
I mean the genre, rented_by, due_date and rating fields.
I'm confused about the meaning of that term "transitive functional depedency". In the book states that I have a transitive functional dependency when: "any of the non-key columns is related to any of the other non-key columns".
I assume that it's because they are related fields somehow, i.e., due_date could be a function of rating fields and the genre, and even of rented_by. A blockbuster movie would be generally be returned earlier than, say, a 60's French cult movie. It's a strange term nevertheless, so wait for a second opinion on this.
That makes sense, thank you =).
If the due_date is a function of genre or rating_fields, then I have a functional dependency. But what about the "transitive" charactheristic?
That very likely has mathematical foundation in the fact that if a is related to b and b is related to c, then a is related to c. Suppose ratings fields depends on genre. Then, if due_date depends on rating fields, due_date also depends on the genre, albeit indirectly.
http://en.wikipedia.org/wiki/Transitive_dependency seems to explain it quite well with books and authors. So for your example, the genre can be determined by the movie title; you could put the relation between movie titles and genres in another table (thus removing the genre from this table) and still find out what the genre is. Depending on what a "movie" is (the concept of certain movie or the physical DVD that can be rented) there may also be a transitive dependency on "due date; presumably different copies of a DVD would have different due dates depending on who rented any one of them.