# Database [3i Infotech Placement]: Sample Questions 128 - 129 of 294

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## Question 128

Database

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### Describe in Detail

Essay▾What are Armstrong rules? How do we say that they are complete and/or sound

### Explanation

- Armstrong՚s Axioms is a set of rules.
- Provides a simple technique for reasoning about functional dependencies.
- Used to infer all the functional dependencies on a relational database.
The well-known inference rules for FD

- Reflexivity:
- If A is a set of attributes and B is a subset of A, then A holds B.

- Augmentation
- If A hold B and C is a set of attributes, then AC holds BC.
- It means that attribute in dependencies does not change the basic dependencies.

- Transitivity
- If A holds B and B holds C, then A holds C.
- If then
- A hold means that A functionally determines B.

- Union
- If A holds B and A holds C, then A holds BC.
- If and

- Pseudo Transitivity
- If A holds B and BC holds D, then AC holds D.
- If

- First three are known as Amstrong Rules.
- They are sound because it is enough if a set of FDs satisfy these three. They are called complete because using these three rules we can generate the rest all inference rules