Database and Expert System Technology - Information Systems Reengineering, Integration and Normalization - page 30

Chapter 2
Database and Expert System Technology

2.1 Hierarchical Model

The hierarchical data model is a logical schema and can be viewed as a subset of
a network model because it imposes a further restriction on the relationship types
in the form of an inverted tree structure. The linkage between record types is in an
automatic fixed set membership. The database access path of a hierarchical data-
base follows the hierarchical path from a parent to child record. The default path is
a hierarchical sequence of top-to-bottom, left-to-right, and front-to-back.
It is common that many real life data can be structured in hierarchical form. For
example, enrollment in a university can be ordered according to the department
organizations. As hierarchies are familiar in nature and in human society, it seems
natural to represent data in a hierarchical structure. Data represent ideas about the
real world that people conceive in terms of entities. Based on the characteristics of
entities, entity type can be defined. Figure 2.1 shows a generic hierarchical tree that
represents entity types where entities refer to record types and record. In the tree,
the record type at the top is usually known as the “root.” Record types are groups
of entities or records that can be described by the same set of attributes. In general,
the root may have any number of dependents; each of these may have any number
of lower-level dependents, and so on, to any number of levels. Individual records
are the actual occurrences of data. The right-hand side is the hierarchical sequence.
There are some important properties of the hierarchical database model.
There is a set of record types (R 1 , R 2 ,… R N ). It is possible to designate a field of
record type as an identifier of a record occurrence of this type. This may provide
either a unique or a nonunique identification. This identifier is called a key.

• Thereisasetofrelationshipsconnectingallrecordtypesinonedatastructure
diagram.
• ThereisnotmorethanonerelationshipbetweenanytworecordtypesR i and R j .
Hence, relationships need not be labeled.
• Therelationshipsexpressedinthedatastructurediagramformatreewithall
edges pointing towards the leaves.