Models of Database Architecture_ Hierarchical, Network and Relational Models

7/23/2019 Models of Database Architecture_ Hierarchical, Network and Relational Models

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11/6/2015 Model s of Dat abase Architecture: Hierarchical, Net work and Relational Models

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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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Models of Database Architecture_ Hierarchical, Network and Relational Models