Chapter 21: Functional Dependencies

Chapter 21: Functional Dependencies

Overview

Functional and multivalued dependencies are the basis for data normalization and keys. They express mappings from one attribute to another attribute in the same relation.

A functional dependency (FD) means that if I know the value of one attribute, I can always determine the value of another. The term ?functional dependency? comes from the behavior of a mathematical function that accepts a value as a parameter and always returns a single result. The notation used in relational theory is an arrow between the two attributes, for example, A B, which can be read as ?A determines B? in English. For example, if I know your employee number, I can determine your name; if I know a part number, I can determine the weight and color of the part; and so forth.

It is important to understand that this relationship cannot be a mere coincidence in the current values in the table. You also need to remember that two different left-hand values might determine the same value on the right-hand side (e.g., a train number determines departure time, but different trains can depart at the same time). A functional dependency is a rule that has to hold for all possible values that could ever be in the table (e.g., to be an employee, you must have an employee number).

The situation where A B and B A can be written as A B with a double-headed arrow. In...

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Chapter 21: Functional Dependencies

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