Learning outcomes for this page: Be aware of the technical definition of Function Dependency

I only include this below for reference. I personally always find it confusing.

“Attribute ‘Y’ in relation [table] ‘R’ is functionally dependent on attribute ‘X’ in ‘R’ if a given value of ‘X’ determines a specific value of ‘Y’, for every instant of time. Either ‘X’ or ‘Y’ may be composite attributes.”

It is written:  X -> Y

I find it confusing because:

• I assume that the ‘specific value of Y’ must be unique which is not the case as shown in the  above examples. Several different values of X can functionally determine the same Y value. Think of the square function –2 x –2 = 4 as does 2 x 2
• Examples often show X as an indexed field in a table but it need not be (see the shipments example at the beginning of this section). The constraint is that when the same X value appears again the Y value must be the same (functionally determined).

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Portfolio exercise:  none   m10|04|00
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

Home > Section 7 > Subsection 7 (Normalisation - part 1)

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