models of database architecture_ hierarchical, network and relational models
TRANSCRIPT
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Models of DatabaseArchitecture:
Hierarchical, Networkand Relational Modelsby Saritha Pujari Database
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Some of the board models of database architecture
are as follows:
The process of defining the conceptual design of data
elements and their inter-relationships is called data
modeling. The traditional applications approach to
data organisation built different models for each data
file.
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Image Cour tesy : ysma.gr /static/images/6_4_DBinput.jpg
Such a diversity of ways in which different data
elements are linked and stored in data files make
these files suitable only for the applications that they
were originally created for. In fact, the details
regarding the exact placement of different data
elements in a file have to be documented very
carefully.
Any change in the order in which various data
elements are placed results in changes in the
application programs using the data file. The database
approach uses a common data model for the entire
database and the user program is not concerned with
the placement of a particular data element. The
database management system (DBMS) acts as an
interface between the database and the user
programs.
The DBMS fetches the data from the database and
makes it available to the user program. This feature
offers the advantage of data independence in the
database approach.
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Conceptually, there are three broad options with
regard to database models. These are:
a. Hierarchical model
b. Network model
c. Relational model
(a) Hierarchical model:
This model presents data to users in a hierarchy of
data elements that can be represented in a sort of
inverted tree. In a sales order processing system, a
customer may have many invoices raised to him andeach invoice may have different data elements. Thus,
the root level of data is customer, the second level is
invoice and the last level is line items such as invoice
number, date, product, quantity, etc.
This structure is quite natural when seen from the
event point of view. However, the lower levels are
owned by higher level data elements, and elements at
the same level have no linkage at all. As a result, the
query such as what products are purchased by which
customer, in the above example, shall be difficult to
carry out in the hierarchical structure.
The query as to which customer purchased which
product would be convenient. Thus, where there are
many-to-many relationships between two entities, this
model would not be appropriate. Figure 9.4 shows the
hierarchical model of data for a sales order processing
application.
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(b) Network model:
In the network model of database, there are no levelsand a record can have any number of owners and also
can have ownership of several records. Thus, the
problem raised above in the sales order processing
will not arise in the network model.
As there is no definite path defined for retrieval of
data, the number of links is very large and thus
network databases are complex, slow and difficult to
implement. In view of the difficulty in
implementation, network model is used only when all
other options are closed.
The typical example of a network database may be the
employee and the department he/she has worked or
can work with in future. Figure 9.5 shows the network
model of data for an employee information system.
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(c) Relational model:
The most recent and popular model of database design
is the relational database model. This model was
developed to overcome the problems of complexity
and inflexibility of the earlier two models in handlingdatabases with many-to-many relationships between
entities.
These models are not only simple but also powerful. In
the relational database, each file is perceived as a flat
file (a two dimensional table) consisting of many lines
(records), each record having key and non-key data
item(s). The key item(s) is the data element(s) that
identifies the record. Figure 9.6 shows the files, and
the fields that each record shall have in a customer
invoicing system.
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In these files, the key data items are customer id,
invoice no, and product code. Each of the files can be
used separately to generate reports. However, data
can also be obtained from any combination of files as
all these files are related to each other with the help of
key data items specified above.
This is the fundamental advantage of the relational
model of database along with its simplicity and the
robustness.
The relational model draws greatly on the work of
E.F. Codd who identifies features of a good
relational database as following:
a) All information is logically represented as tables
and the access of data are possible by the names of
fields. Thus, the order, position or file linkage is not a
matter of concern for users.
b) The data dictionary has information regarding the
database structure including the data type; size, etc.,
definitions, relationships and access permissions. The
authorised users can learn about the database
environment and change the environment using the
data description language (DDL).
c) A data manipulation language (DML) is available to
users including programmers for creation, insertion,
modification, retrieval, organising and deletions of
any part of the database. These manipulations are
possible at the record level as well as for the whole
file, giving the flexibility in defining access
permissions for various categories of users.
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d) Any modification in the structure of database in
terms of splitting the table horizontally or vertically
should not have any impact on the logic of the
program using the database. This data independence
is the core advantage of the relational model of
database.
e) The distributed independence of data is another
feature of a good relational database. The user
programs do not require any change when data are
first distributed or redistributed. The actual physical
location of data does not matter to the user so long as
that field appears in the data dictionary as local.
As may be noted from Fig. 9.6, none of the fields is
common in any two files except the key item. So, the
data redundancy can be avoided in this model. For
this purpose, a process of data normalisation is
undertaken while designing the structure of a
database.
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