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Home > Section 7 > Subsection 7 (Normalisation - part 1) |    |
Learning outcomes for this page:
1. Be able to describe what Composite
non-atomic values are.
2. Be able to describe what Multivalued
non-atomic values (repeating groups) are
3. Be able to provide a definition of
first normal form
A definition of First Normal Form:
“A table is said to be in first normal form if every field it contains only possesses atomic values”
Why should all fields in a database only consist of atomic values? The concept of atomic values was briefly discussed in the section introducing data and information (See: http://www.robinbt2.free-online.co.uk/virtualclassroom/contents.htm section 5.2). An atomic value is one that is indivisible within the context of a database field definition (e.g. integer, real, code of some sort etc.) Field values that are not atomic are of two undesirable types (Elmasri & Navathe 1989 p.139,41):
Undesirable - non atomic field types:
Composite
Multivalued
Composite fields are those that can be broken down within the database context. Examples include; a full name (a composite of forename and surname), Address (a composite of road, town country etc,), and BP (a composite of two integers).
Composite fields are 'fixed' (or “structured” if you want to use the more correct term), by breaking them up into a sensible number of fields. For example the 'full name' field above might be split into a sensible number of fields (depending upon the country).
Most fields are single-valued. That is a field takes a single value for each record. Examples include; date of birth, sex, and marital status. In contrast a field which may take more than one value for each record and is said to be multi-valued. Examples include; a list of qualifications for an employee, the number of x-rays a patient has undergone, the number of instruments used in an operation or the number of times a patient visits a doctor. Multi-valued field values are also called 'repeating groups' (Date 1995 p56). Each repeating group may consist of one or more repeating attributes. To discover how these multi-valued fields are fixed we will consider this last example in more depth.
For: Clinicians | NHS managers | Non healthcare workers
Written by Robin Beaumont & Chris Noden e-mail:robin@ieg-net.co.uk or chrisnoden@hotmail.com