Databases Reference
In-Depth Information
Taxonomies
Originally, the word “taxonomy” referred to a method of clas-
sification used in biology, and introduced into that science in the
18
th
century by Carl Linnaeus. Taxonomy in biology began as a
system of classification based on morphological similarities
and differences among groups of living things. But with the
modern synthesis of Darwinian evolutionary theory, Mendelian
genetics, and the Watson-Crick discovery of the molecular basis
of life and its foundations in the chemistry of DNA, biological
taxonomy has, for the most part, become a system of classifica-
tion based on common genetic ancestry.
Partitioned Semantic Trees
As borrowed by computer scientists, the term “taxonomy”
refers to a partitioned semantic tree. A tree structure is a hierar-
chy, which is a set of non-looping (
acyclic
) one-to-many
relationships. In each relationship, the item on the “one” side is
called the
parent
item in the relationship, and the one or more
items on the “many” side are called the
child
items. The items
that are related are often called
nodes
of the hierarchy.
Continuing the arboreal metaphor, a tree consists of one root
node (usually shown at the top of the structure, and not, as the
metaphor would lead one to expect, at the bottom), zero or more
branch nodes, and zero or more leaf nodes on each branch. This
terminology is illustrated in
Figure 2.1
.
Tree structure
. Each taxonomy is a hierarchy. Therefore,
except for the root node, every node has exactly one parent
node. Except for the leaf nodes, unless the hierarchy consists of
Party
root node
branch node
Person
Organization
Supplier
Self
Customer
leaf nodes
Figure 2.1 An Illustrative Taxonomy.
