Database and Expert System Technology - Information Systems Reengineering, Integration and Normalization - page 36

SYSTEM

set set set

Course Department Student

Course#
coure-location

dept#
dept-name

student#
s-name

set set

Instructor Grade

set

set

grade

inst-name
inst-addr

set set

Prerequisite

Section

Prerequisite#
prerequisite-title

section#

Fig. 2.3 A Network model for unversity enrollment

2.2 Network (CODASYL) Model

The Network model is a logical schema and is based on tables and graphs (CO-
DASYL 1971 ). The nodes of a graph (segment types) usually correspond to the
entity types, which are represented as connections (sets) between tables in the form
of network. The insertion and retention of segment types depend on the set member-
ship constraints that exist between the owner and member segments, with automatic
or manual insertion, and fixed, mandatory, or optional retention.
A network database model is similar to a hierarchical database model that rep-
resents data and a data relationship in a graphical form. The network model differs
from the hierarchical model as:

• Therecanbemorethanoneedgebetweenagivenpairofentities
• Thereisnoconceptofrootand
• Asegmentcanhavemorethanoneparentsegment

For example, Fig. 2.3 is a network model for the university enrollment system.