Database and Expert System Technology - Information Systems Reengineering, Integration and Normalization - page 70

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Arms Mammal

has_a is_a

is_a

Human

is_a

Jack Ape

has_a

is_a

has_a

Learning
skill

One-Arm-
Human

generate

Knowledge

is_a has_a

Lee Arms number-of 1

Fig. 2.15 A semantic network

hierarchical structure for categorizing classes of real world objects (see Fig. 2.15 ).
An object in the semantic network is called a node. Nodes are connected by arcs or
links. Ordinarily, both the nodes and the links are labeled.
Nodes are used to represent physical objects, conceptual entities, or descriptors.
Physical objects can be seen or touched (e.g., human, ape, etc.). Conceptual entities
are objects such as acts, events, or abstract categories, like mammal, 2, and so on.
Descriptors provide additional information about objects (e.g., “knowledge” stores
information about “human”).
Links are used to represent the relationships between nodes. Examples of rela-
tionships include IS_A, HAS_A, and human-defined relationships. The IS_A link
is often used to represent the class/instance relationship. For example, “Jack IS_A
Human” or “Human IS_A Mammal.” The IS_A link is, also, used for the purpose of
generalization. It is used to provide inference using property inheritance deduction
and organization in a generalization hierarchy. Inheritance has become an important
feature of semantic networks. It refers to the ability of one node to “inherit” char-
acteristics from other related nodes. Property inheritance means that instances of a
class are assumed to have all of the properties of the more general classes of which
they are members.
HAS_A links identify nodes that are properties of other nodes. For example,
“Human HAS_A Knowledge” or “Human HAS_A two Legs.” The HAS_A link has
thus often been used for aggregation. It is the same as the A_PART_OF relation-
ship that represents a situation where one class is an assembly (or aggregate) of
component objects in a database application. Aggregation is one important feature
of the semantic network by which the relevant facts about objects or concepts can