In this model, data is represented as
tree structures in a hierarchy of records, as shown on the right in Figure 1-1. IBM??™s
product based on this model was brought to market in 1969 as the Information Management
System (IMS). As recently as 1980, almost all database implementations
used either the network or hierarchical approach. Although several competitors sold
similar technologies around 1980, only IMS could still be found in many large organizations
20 years later.
Figure 1-1. Network model (left) and hierarchical model (right)
4 | Chapter 1: Introducing Oracle
Relational Basics
The relational database uses the concept of linked two-dimensional tables consisting
of rows and columns, as shown in Figure 1-2. Unlike the hierarchical approach, no
predetermined relationship exists between distinct tables. This means that data
needed to link together the different areas of the network or hierarchical model need
not be defined. Because relational users don??™t need to understand the representation
of data in storage to retrieve it (and many such users create ad hoc queries), ease of
use helped popularize the relational model.
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