Functional Dependencies Flashcards
What does a functional dependency describe?
The relationship between attributes
What is a functional dependency that holds?
When in a relation R, the relationship can be described using a functional dependency. However, these dependencies may not hold for all instances of relation R
What is a trivial functional dependency?
a -> B is trivial if (B subset or == A)
What is a partial functional dependency?
A partial functional dependency exists if you can remove an attribute from a and the dependency a -> B still exists.
What is a full functional dependency?
B is fully functionally dependent on a if removing any attribute from a results in the dependency no longer existing.
What is a transitive dependency?
Describes a condition where A, B and C are sets of attributes of a relation such that if A -> B and B -> then
A->B transitively
Closures are important, why?
Given a set of functional dependencies, F, there are certain other functional dependences that are logically implied by F. This is important in normalization
How do we define reflexivity?
If B (subset or ==) a, then a -> b
How do we define augmentation?
if a -> B, then ya -> yb
What is a formal definition of a closure?
If a -> B is in F+ then B (subset or eqaul to) a+
