part Introduction to Databases This page intentionally left blank chapter 1 Databases and Database Users D atabases 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 bib- liographic 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.1 we start by defin- ing a database, and then we explain other basic terms.2, we provide a 3 4 Chapter 1 Databases and Database Users simple UNIVERSITY database example to illustrate our discussion.3 describes some of the main characteristics of database systems, and Sections 1.5 categorize the types of personnel whose jobs involve using and interacting with database systems.8 offer a more thorough discussion of the various capabilities provided by database systems and discuss some typical database applications.9 summarizes the chapter.
The reader who desires a quick introduction to database systems can study Sections 1.5, then skip or browse through Sections 1.8 and go on to Chapter 2.1 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, medi- cine, 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- 1We will use the word data as both singular and plural, as is common in database literature; the context will determine whether it is singular or plural. In standard English, data is used for plural and datum for singular.1 Introduction 5 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.
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.
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 6 Chapter 1 Databases and Database Users 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.1 illustrates some of the concepts we have dis- cussed so far.2 An Example Let us consider a simple example that most readers may be familiar with: a UNIVERSITY database for maintaining information concerning students, courses, and grades in a university environment.2 shows the database structure and a few sample data for such a database. The database is organized as five files, each of which stores data records of the same type.3 The STUDENT file stores data on each student, the COURSE file stores data on each course, the SECTION file stores data on each section of a course, the GRADE_REPORT file stores the grades that students receive in the various sections they have completed, and the PREREQUISITE file stores the prerequisites of each course. To define this database, we must specify the structure of the records of each file by specifying the different types of data elements to be stored in each record.2, each STUDENT record includes data to represent the student’s Name , Student_number, Class (such as freshman or ‘1’, sophomore or ‘2’, and so forth), and 2The term query, originally meaning a question or an inquiry, is loosely used for all types of interactions with databases, including modifying the data.
3We use the term file informally here. At a conceptual level, a file is a collection of records that may or may not be ordered.2 An Example 7 Users/Programmers Database System Application Programs/Queries DBMS Software Software to Process Queries/Programs Software to Access Stored Data Stored Database Definition Stored Database (Meta-Data) Figure 1.1 A simplified database system environment. Major (such as mathematics or ‘MATH’ and computer science or ‘CS’); each COURSE record includes data to represent the Course_name, Course_number, Credit_hours, and Department (the department that offers the course); and so on. We must also specify a data type for each data element within a record.
For example, we can specify that Name of STUDENT is a string of alphabetic characters, Student_number of STUDENT is an integer, and Grade of GRADE_REPORT is a single character from the set {‘A’, ‘B’, ‘C’, ‘D’, ‘F’, ‘I’}. We may also use a coding scheme to rep- resent the values of a data item. For example, in Figure 1.2 we represent the Class of a STUDENT as 1 for freshman, 2 for sophomore, 3 for junior, 4 for senior, and 5 for graduate student. To construct the UNIVERSITY database, we store data to represent each student, course, section, grade report, and prerequisite as a record in the appropriate file.
Notice that records in the various files may be related.