THE DATABASE APPROACH - Database Design and Development: An Essential Guide for IT Professionals - page 24

Root
Segment

Parent
Segment CUSTOMER

Child
Segment

ORDER

Parent
Segment

ORDER
LINE ITEM Child

Segment

Relationship links
through physical
pointers

Figure 1-8 Hierarchical data model.

Physical Pointers. How are the orders of a particular customer linked in the
implementation of the hierarchical data model? These linkages are by means of
physical pointers or physical storage addresses embedded within physical records
in the database. Physical pointers link records of the parent segments to those of
the child segments by means of parent-child forward or backward pointers. Simi-
larly, forward and backward physical pointers link records of the same segment type.

Network

The hierarchical data model represents well any business data that inherently con-
tains levels one below the other. We have just discussed how the manufacturing
application deals with hierarchical levels of plant inventory with assemblies broken
down into lower-level components. The hierarchical data model suits this applica-
tion well. However, in the real world, most data structures do not conform to a
hierarchical arrangement. The levels of data structures do not fall into nice depen-
dencies one below another as in a hierarchy. In the hierarchical data model,
you have noticed that each data segment at any level can have only one parent at
the next higher level. In practice, many sets of related elements may not be sub-
jected to such restrictions.
Let us consider a common set of related data elements in a typical business. The
data elements pertain to customers placing orders and making payments, salesper-
sons being assigned, and salespersons being part of sales territories. All of these data
elements cannot be arranged in a hierarchy. The relationships cross over among the
data elements as though they form a network. Refer to Figure 1-9 and note how it
represents a network arrangement and not a hierarchical arrangement. Observe the
six data elements of sales territory, salesperson, customer, order, order line item, and
payment as nodes in a network arrangement.