DATABASE DESIGN

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# DATABASE DESIGN - PowerPoint PPT Presentation

DATABASE DESIGN. Functional Dependencies. Overview. Functional Dependencies Normalization Functional dependencies Normal forms. Functional Dependencies FD. Describes relationship between attributes in a relation.

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### DATABASE DESIGN

Functional Dependencies

Overview
• Functional Dependencies
• Normalization
• Functional dependencies
• Normal forms
Functional Dependencies FD
• Describes relationship between attributes in a relation.
• If two tuples of a table agree on their X value they also agree on their Y value
• If A and B are attributes of relation R, B is functionally dependent on A (denoted A B), if each value of A in R is associated with exactly one value of B in R.
• Define all the functional dependencies for the employee data-items
FD Rules
• If X and Y sets of attributes contained in HEAD(T), AND then X Y
• Trivial Dependency - FD of the form X Y that hold for any T where X, Y HEAD(T)
• example - right hand side subset of left side
Armstrong’s Axioms +
• Reflexivity: If Y is a subset of X, then X Y
• Self-determination: Y Y
• Transitivity: If X Y and Y Z, the X Y
• Augmentation: If X Y, then XZ YZ
• Union: If X Y and X Z, then X YZ
• Decomposition: If X YZ, then X Y and X Z
• Pseudotransivity: If X Y and WY Z, then XW Z
• Accumulation: If X YZ and Z BW, then X YZB
Irreducible Sets of FDs
• Set, S of FD irreducible iff
• right-hand side of every FD in S singleton set
• left-hand side of every FD in S irreducible in turn (no determinant can be discarded from the determinant without loosing information
• No FD can be discarded from S without changing closure (loosing information)
Review Question
• Consider the following two sets of functional dependencies_

F = (A C, AC D, E AD, E H)

G = (A CD, E AH)

Check whether they are equivalent.