Databases Reference
In-Depth Information
models in one database. The relational database approach became commercially
viable in about 1980. It is currently the preferred database model and is imple-
mented in applications from simple desktop storage to massive enterprise-level
applications. The object-oriented approach has proven useful for a variety of
niche applications. In this approach, data items are treated as objects, which are
individual items that can be defined and described. The importance of the object-
oriented approach is that some of the key object-oriented database concepts have
found their way into some of the mainstream relational systems and provide a
hybrid object/relational database system.
2.2.1 The Hierarchical Database Model
The
hierarchical database model
is based on a hierarchical data organization.
In the business world, many data structures are hierarchical in nature. You can
trace a hierarchy by division, subdivision, department, and employee. Figure 2-4
illustrates one such model showing the hierarchy of customer, order, and order
line item. A customer can have one or more orders, and an order can have one
or more line items.
Each data structure representing a business object is at one hierarchical level.
The data segment at the top level of the hierarchy is known as the root data seg-
ment (as in an inverted tree). The relationship between each pair of data struc-
tures at levels next to each other is a parent-child relationship. CUSTOMER is
a parent data segment whose child is ORDER. Each child can have one parent,
Figure 2-4
Root
Segment
Parent
Segment
CUSTOMER
Child
Segment
ORDER
Parent
Segment
ORDER
LINE ITEM
Child
Segment
Relationship links
through physical
pointers
Hierarchical database model.
