Functional Dependencies

Question 1

what is a functional dependency

Answer 1

a constraint between two sets of attributes in a relation

Question 2

X -> Y means

Answer 2

X determines Y

Question 3

what are the advantages of functional dependencies (6)

Answer 3

avoids data redundancy

maintain quality

define meaning and constraints

identify bad designs

find facts

determine the key

Question 4

give armstrong’s axioms (3)

Answer 4

reflexivity (ABC -> AB)

augmentation (add something to both sides)

transitivity (A->B, B->C, A->C)

Question 5

give the inference rules derived from the axioms

Answer 5

union A->B, A->C, A->BC

pseudo transitivity (substitution. A->B, CB->D, AC->D)

decomposition (A->BC means A->B, A->C)

Question 6

what is F+

Answer 6

F closure

All the functional dependencies that could possibly be implied

Question 7

what is X+

Answer 7

X closure

we can calculate X+ or A+ etc to determine the key