Functional Dependency

[Bukwee]

Can anyone explain Functional Dependency in plain English? I'm starring at a table which looks like this. How can you simply describe Functional Dependency and how it can relate to a real world situation?

A B C
=======
a1 b1 c1
a1 ba c1
a2 b1 c1
a2 ba c3

Thank you!

Truly confused

 

[tlbroadbent]

Functional dependency is math term for the key relationship between data elements or attributes. A data element (B) that is functionally dependent on another data element (A) will always exist in a relation (or in database terms, a table) such that a unique value for A will always "determine" or "define a unique value of" B. In other words, if I search for A, I will always get 1 and onlt 1 B.

In your example, the is no functional dependency because there is no attribute that is dependent on another. However, functional dependency is really defined by the schema not the data.

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Mike Chapple provides a simple, plain English definition at About.com.

Definition: A functional dependency occurs when one attribute in a relation uniquely determines another attribute. This can be written A -> B which would be the same as stating "B is functionally dependent upon A."

Examples: In a table listing employee characteristics including Social Security Number (SSN) and name, it can be said that name is functionally dependent upon SSN (or SSN -> name) because an employee's name can be uniquely determined from their SSN. However, the reverse statement (name -> SSN) is not true because more than one employee can have the same name but different SSNs.

Source:

http://databases.about.com/compute/databases/library/glossary/bldef-functdep.htm

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There is a very good academic treatment of the topic at the following URL.

http://vision.ucsd.edu/~deborah/132b/fd/fdDef.html

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Also check the following.

http://cs.wwc.edu/~aabyan/415/FunDep.html

Terry

http://members.home.net/tlbroadbent/index.htm