Database-MySQL [3i Infotech Placement]: Sample Questions 124 - 125 of 162

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Question 124

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Describe in Detail

Essay▾

What do you understand by dependency preservation?

Explanation

Understanding of Dependency Preservation
  • A decomposition of a relation R into is dependency preserving decomposition with respect to the set of functional dependencies F that hold on R only if the following is hold:

    Where,

    - Sets of functional dependencies of relations .

    - Closure of union of all sets of functional dependencies.

    - Closure of set of functional dependency

    • If the closure of set of functional dependencies of individual relations are equal to the set of functional dependencies of the main relation R, then we would say the decomposition D is lossless dependency preserving decomposition.
  • Example:

    Decomposition of

    Does this decomposition preserve the given dependencies?

  • Solution:

In the following dependencies hold:

In the following dependencies hold:

In all the original dependencies occur,

So, this decomposition preserves dependencies.

Question 125

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Describe in Detail

Essay▾

What is Functional Dependency?

Explanation

Functional Dependency
  • A functional dependency (FD) on a relation schema R is a constraint X ⇾ Y, where X and Y are subsets of attributes of R.
  • An FD is a relationship between an attribute “Y” and a determinant (1 or more other attributes) “X” such that for a given value of a determinant the value of the attribute is uniquely defined.
    • X is a determinant
    • X determines Y
    • Y is functionally dependent on X
    • X Y
    • X Y is trivial if Y X

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