FUNDAMENTALS OF
Database Systems
BY Ramez Elmasri, Shamkant B. Navathe
Chapter 1
Databases
and Database Users
Databases and database systems are an essential component of life in
modern society: most of us encounter several activities every day that involve
some interaction with a database.
For example, if we go to the bank to deposit or withdraw funds, if we
make a hotel or airline reservation, if we access a computerized library
catalog to search for a bibliographic item, or if we purchase something
online—such as a book, toy, or com-puter—chances are that our activities will
involve someone or some computer program accessing a database. Even purchasing
items at a supermarket often auto-matically updates the database that holds the
inventory of grocery items.
These interactions are examples of what we may call traditional database applica-tions, in which most of the
information that is stored and accessed is either textual or numeric. In the past few years, advances in technology have led
to exciting new applications of database systems. New media technology has made
it possible to store images, audio clips, and video streams digitally. These
types of files are becom-ing an important component of multimedia databases. Geographic
information systems (GIS) can
store and analyze maps, weather data, and satellite images. Data warehouses and online
analytical processing (OLAP) systems are used in many companies to extract and analyze useful business information from
very large data-bases to support decision making. Real-time and active
database technology is used to control industrial and manufacturing
processes. And database search tech-niques are being applied to the World Wide
Web to improve the search for informa-tion that is needed by users browsing the
Internet.
To understand the fundamentals of database technology, however, we must
start from the basics of traditional database applications. In Section 1.1 we
start by defin-ing a database, and then we explain other basic terms. In
Section 1.2, we provide a simple UNIVERSITY database
example to illustrate our discussion. Section 1.3 describes some of the main
characteristics of database systems, and Sections 1.4 and 1.5 categorize the
types of personnel whose jobs involve using and interacting with database
systems. Sections 1.6, 1.7, and 1.8 offer a more thorough discussion of the
various capabilities provided by database systems and discuss some typical
database applications. Section 1.9 summarizes the chapter.
The reader who desires a quick introduction to database systems can
study Sections 1.1 through 1.5, then skip or browse through Sections 1.6
through 1.8 and go on to Chapter 2.
Introduction
Databases and database technology have a major impact on the growing use
of computers. It is fair to say that databases play a critical role in almost
all areas where computers are used, including business, electronic commerce,
engineering, medicine, genetics, law, education, and library science. The word
database is so com-monly used that we
must begin by defining what a database is. Our initial definition is quite
general.
A database is a collection of
related data.1 By data, we mean known facts
that can be recorded and that have implicit meaning. For example, consider the
names, tele-phone numbers, and addresses of the people you know. You may have
recorded this data in an indexed address book or you may have stored it on a hard
drive, using a personal computer and software such as Microsoft Access or
Excel. This collection of related data with an implicit meaning is a database.
The preceding definition of database is quite general; for example, we
may consider the collection of words that make up this page of text to be
related data and hence to constitute a database. However, the common use of the
term database is usually more
restricted. A database has the following implicit properties:
A database represents some aspect
of the real world, sometimes called the miniworld
or the universe of discourse (UoD).
Changes to the miniworld are
reflected in the database.
A database is a logically
coherent collection of data with some inherent meaning. A random assortment of
data cannot correctly be referred to as a database.
A database is designed, built,
and populated with data for a specific purpose. It has an intended group of
users and some preconceived applications in which these users are interested.
In other words, a database has some source from which data is derived,
some degree of interaction with events in the real world, and an audience that
is actively inter ested in its contents. The end users of a database may
perform business transactions (for example, a customer buys a camera) or events
may happen (for example, an employee has a baby) that cause the information in
the database to change. In order for a database to be accurate and reliable at
all times, it must be a true reflection of the miniworld that it represents;
therefore, changes must be reflected in the database as soon as possible.
A database can be of any size and complexity. For example, the list of
names and addresses referred to earlier may consist of only a few hundred
records, each with a simple structure. On the other hand, the computerized
catalog of a large library may contain half a million entries organized under
different categories—by pri-mary author’s last name, by subject, by book
title—with each category organized alphabetically. A database of even greater
size and complexity is maintained by the Internal Revenue Service (IRS) to
monitor tax forms filed by U.S. taxpayers. If we assume that there are 100
million taxpayers and each taxpayer files an average of five forms with
approximately 400 characters of information per form, we would have a database
of 100 × 106 × 400 × 5 characters (bytes) of information. If the IRS keeps the past three
returns of each taxpayer in addition to the current return, we would have a
database of 8 × 1011 bytes (800 gigabytes). This huge amount of information must be
organized and managed so that users can search for, retrieve, and update the
data as needed.
An example of a large commercial database is Amazon.com. It contains
data for over 20 million books, CDs, videos, DVDs, games, electronics, apparel,
and other items. The database occupies over 2 terabytes (a terabyte is 1012 bytes worth of stor-age) and is
stored on 200 different computers (called servers). About 15 million vis-itors
access Amazon.com each day and use the database to make purchases. The database
is continually updated as new books and other items are added to the inventory
and stock quantities are updated as purchases are transacted. About 100 people
are responsible for keeping the Amazon database up-to-date.
A database may be generated and maintained manually or it may be
computerized. For example, a library card catalog is a database that may be
created and maintained manually. A computerized database may be created and maintained
either by a group of application programs written specifically for that task or
by a database management system. We are only concerned with computerized
databases in this book.
A database management system
(DBMS) is a collection of programs that enables users to create and
maintain a database. The DBMS is a general-purpose
software sys-tem that facilitates the processes of defining, constructing, manipulating, and sharing databases among various users and applications. Defining a database involves
spec-ifying the data types, structures, and constraints of the data to be
stored in the data-base. The database definition or descriptive information is
also stored by the DBMS in the form of a database catalog or dictionary; it is
called meta-data. Constructing the database is the
process of storing the data on some storage medium that is con-trolled by the
DBMS. Manipulating a database
includes functions such as querying the database to retrieve specific data,
updating the database to reflect changes in the miniworld, and generating
reports from the data. Sharing a
database allows multi-ple users and programs to access the database
simultaneously.
An application program
accesses the database by sending queries or requests for data to the DBMS. A query2 typically causes some data to be
retrieved; a transaction may cause
some data to be read and some data to be written into the database.
Other important functions provided by the DBMS include protecting the database and maintaining it over a long period of
time. Protection includes system protection against hardware or
software malfunction (or crashes) and security
protection against unauthorized or malicious access. A typical large
database may have a life cycle of many years, so the DBMS must be able to maintain the database system by
allowing the system to evolve as requirements change over time.
It is not absolutely necessary to use general-purpose DBMS software to
implement a computerized database. We could write our own set of programs to
create and maintain the database, in effect creating our own special-purpose DBMS software. In either
case—whether we use a general-purpose DBMS or not—we usually have to deploy a
considerable amount of complex software. In fact, most DBMSs are very complex
software systems.
To complete our initial definitions, we will call the database and DBMS
software together a database system.
Figure 1.1 illustrates some of the concepts we have discussed so far.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.