Armstrong's Axioms
Introduction to Axioms Rules
- Armstrong's Axioms is a set of rules.
- It provides a simple technique for reasoning about functional dependencies.
- It was developed by William W. Armstrong in 1974.
- It is used to infer all the functional dependencies on a relational database.
Various Axioms Rules
A. Primary Rules
Rule 1 | Reflexivity If A is a set of attributes and B is a subset of A, then A holds B. { A → B } |
Rule 2 | Augmentation If A hold B and C is a set of attributes, then AC holds BC. {AC → BC} It means that attribute in dependencies does not change the basic dependencies. |
Rule 3 | Transitivity If A holds B and B holds C, then A holds C. If {A → B} and {B → C}, then {A → C} A holds B {A → B} means that A functionally determines B. |
B. Secondary Rules
Rule 1 | Union If A holds B and A holds C, then A holds BC. If{A → B} and {A → C}, then {A → BC} |
Rule 2 | Decomposition If A holds BC and A holds B, then A holds C. If{A → BC} and {A → B}, then {A → C} |
Rule 3 | Pseudo Transitivity If A holds B and BC holds D, then AC holds D. If{A → B} and {BC → D}, then {AC → D} |
Sometimes Functional Dependency Sets are not able to reduce if the set has following properties,
1. The Right-hand side set of functional dependency holds only one attribute.
2. The Left-hand side set of functional dependency cannot be reduced, it changes the entire content of the set.
3. Reducing any functional dependency may change the content of the set.
A set of functional dependencies with the above three properties are also called as
Canonical or Minimal.Trivial Functional Dependency
Trivial | If A holds B {A → B}, where A is a subset of B, then it is called a Trivial Functional Dependency. Trivial always holds Functional Dependency. |
Non-Trivial | If A holds B {A → B}, where B is not a subset A, then it is called as a Non-Trivial Functional Dependency. |
Completely Non-Trivial | If A holds B {A → B}, where A intersect Y = Φ, it is called as a Completely Non-Trivial Functional Dependency. |
Example:
Consider relation E = (P, Q, R, S, T, U) having set of Functional Dependencies (FD).
P → Q P → R
QR → S Q → T
QR → U PR → U
Calculate some members of Axioms are as follows,
1. P → T
2. PR → S
3. QR → SU
4. PR → SU
Solution:
1. P → T
In the above FD set, P → Q and Q → T
So, Using Transitive Rule: If {A → B} and {B → C}, then {A → C}
∴ If P → Q and Q → T, then
P → T.
P → T
2. PR → S
In the above FD set, P → Q
As, QR → S
So, Using Pseudo Transitivity Rule: If{A → B} and {BC → D}, then {AC → D}
∴ If P → Q and QR → S,
then PR → S.
PR → S
3. QR → SU
In above FD set, QR → S and QR → U
So, Using Union Rule: If{A → B} and {A → C}, then {A → BC}
∴ If QR → S and QR → U,
then QR → SU.
QR → SU
4. PR → SU
So, Using Pseudo Transitivity Rule: If{A → B} and {BC → D}, then {AC → D}
∴ If PR → S and PR → U,
then PR → SU.
PR → SU