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

Define Armstrong Rules
  • 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

Question 129

Database
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Write in Short

Short Answer▾

What is the difference between “NULL in C” and “NULL in Oracle?”

Explanation

  • The NULL in C treated as Zero or void.
  • But in SQL NULL value is Non or blank represented it can՚t manipulated.

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