Functional Dependency (FD)
Learning outcomes for this page:
1. Be able to provide a definition of Functional Dependency
(FD)
2. Be aware of mapping diagrams to help decide if
two fields are Functionally Dependent
The above discussion was concerned with the dependencies between tables. However, in contrast normalisation is concerned with the dependencies within a particular table.
Dependency, in this context, means that the value of one field is determined by a value in another field in the same record of a table.
Why the word ‘functional’ I’m not sure if this is correct, but as I understand it is it because the dependency is actually a function of the 1:1 or N:1 variety (Carter p34). This is demonstrated below. Actually Codd used the term ‘internal dependence’ in his first paper (Date 1999).
Quoting Date (1995 p271) who discusses a shipment table that consists of three fields (supplier, part, quantity):
“Basically, a functional dependence (usually abbreviated FD) is a many-to one relationship from one set of attributes to another within a given relation [‘table’]. In the shipments relation SP [opposite], for example, there is a functional dependence from the set of attributes {S#,P#} to the set of attributes {QTY}. What this means is that for many values of the attribute pair S#-P# there is one corresponding value of the attribute QTY.”
To illustrate the above point concerning the fact that many values of the attribute pair {S#,P#} go to one {QTY] value I have drawn a set of lines based upon the particular {S#,P#} value to the associated QTY value. For example {S1,P1},{S2,P1} and {S4,P4} are all associated with the QTY value 300. I call this a Mapping diagram.
 |
 |
In general we have the situation shown in the mapping diagram opposite. |
On the following page we will consider a more familiar example from the Access practical sessions.
Portfolio exercise: none
Time: 0 minutes
For: Clinicians | NHS managers | Non healthcare workers
Written by Robin Beaumont & Chris Noden e-mail:robin@ieg-net.co.uk or chrisnoden@hotmail.com
