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2 CHAPTER 1 Introduction
expanded these defi nitions: In a relational database, a data item is called a column
or attribute ; a record is called a row or tuple ; and a fi le is called a table .
A database is a more complex object. It is a collection of interrelated stored
data — that is, interrelated collections of many different types of tables — that serves
the needs of multiple users within one or more organizations. The motivations for
using databases rather than fi les include greater availability to a diverse set of users,
integration of data for easier access to and updating of complex transactions, and
less redundancy of data.
A database management system (DBMS) is a generalized software system for
manipulating databases. A DBMS supports a logical view (schema, subschema);
physical view (access methods, data clustering); data defi nition language; data
manipulation language; and important utilities, such as transaction management
and concurrency control, data integrity, crash recovery, and security. Relational
database systems, the dominant type of systems for well-formatted business data-
bases, also provide a greater degree of data independence than the earlier hierar-
chical and network (CODASYL) database management systems. Data independence
is the ability to make changes in either the logical or physical structure of the
database without requiring reprogramming of application programs. It also makes
database conversion and reorganization much easier. Relational DBMSs provide a
much higher degree of data independence than previous systems; they are the
focus of our discussion on data modeling.
1.2 THE DATABASE LIFE CYCLE
The database life cycle incorporates the basic steps involved in designing a global
schema of the logical database, allocating data across a computer network, and
defi ning local DBMS-specifi c schemas. Once the design is completed, the life cycle
continues with database implementation and maintenance. This chapter contains
an overview of the database life cycle, as shown in Figure 1.1 . The result of
each step of the life cycle is illustrated with a series of diagrams in Figure 1.2 .
Each diagram shows a possible form of the output of each step, so the reader
can see the progression of the design process from an idea to actual database
implementation.
I. Requirements analysis. The database requirements are determined by inter-
viewing both the producers and users of data and using the information to
produce a formal requirements specifi cation. That specifi cation includes the
data required for processing, the natural data relationships, and the software
platform for the database implementation. As an example, Figure 1.2 (step I)
shows the concepts of products, customers, salespersons, and orders being
formulated in the mind of the end user during the interview process.
II. Logical design. The global schema , a conceptual data model diagram that
shows all the data and their relationships, is developed using techniques such