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3. Dependency on parent node, which is the root node, and its deletion can cause deletion of all
child nodes and, in turn, the entire model.
4. Changes in the structure require changes in all the applications.

IBM’s IMS (Information Management System) and system2000 are examples of hierarchical
database management system.
12.3.1.2 Network Data Model

The network model is a database model conceived as a flexible way of representing objects
and their relationships. It consists of a collection of records connected to one another through
links. The network model replaces the hierarchical model with a graph, thus allowing multiple
connections among the nodes. The major point of difference between the two models is that unlike
the hierarchical model, the network model provides the capability to handle many-to-many (M:M)
relationships. Thus, a child can have more than one parent.

Fig. 12.9: Network Model

From Fig.12.9, it is clear that this model represents many-to-many relationships. It can be observed
that the file EXAM is associated with three tables, viz. LIBRARY, FEES and STUDENT. Thus, the
figure shows that a child can have more than one parent. This model is an improved version of
hierarchical model having records along with pointers. These pointers establish many-to-many
relationships and are termed as Links. Each record has its respective pointer or link with which it
is associated.
The advantages of network model are:
1. It can handle more relationship types, i.e., M:M (many-to-many) multi-parent relationships.
2. Data access is easy and more flexible.
3. It includes DDL (Data Definition Language) as well as DML (Data Manipulation Language).
4. It promotes better data integrity.
Relational Database and SQL
The limitations of this model are:
1. The system is quite complicated in structure since there are several number of links which also
limits efficiency.
2. Structural changes require changes to be made in all application programs.
3. A high-level language interface is required to interact with the database.
4. Extra memory is consumed for holding links (pointers).

UNIVAC’s DMS 1100 and DBMS 10-20 from DEC are examples of network database management
system.
12.7