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Home > Section 7 > Subsection 7 (Normalisation - part 1) |    |
Learning outcomes for this page:
Be aware of the restrictions of inferring
dependency from inspecting database tables
It is often tempting to ‘guess’ a functional dependency from inspecting a table rather than discussing the data with domain experts. For example inspecting the table below concerning Teachers, Courses and Texts (based on Elmasri & Navathe 1989 p367) we may be tempted to say that TEXT functionally determines COURSE (i.e. course is dependent upon text) unfortunately we cannot be sure unless we know this always holds.
It would be sufficient to find a single counterexample to disprove the functional dependency. For example from the table below we can say that TEACHER does not functionally determine COURSE because we have the counter example of Whatling running both Database and Robotics courses.
|
Teacher |
Course |
Text |
|
Whatling |
Databases |
Codd |
|
Whatling |
Robotics |
Shermann |
|
Jones |
Consumer informatics |
Musan |
|
Brown |
Databases |
Date |
For: Clinicians | NHS managers | Non healthcare workers
Written by Robin Beaumont & Chris Noden e-mail:robin@ieg-net.co.uk or chrisnoden@hotmail.com