Datasheet

9
Chapter 1: Relational Database Fundamentals
What Is a Database?
The term database has fallen into loose use lately, losing much of its original
meaning. To some people, a database is any collection of data items (phone
books, laundry lists, parchment scrolls . . . whatever). Other people define
the term more strictly.
In this book, I define a database as a self-describing collection of integrated
records. And yes, that does imply computer technology, complete with
programming languages such as SQL.
A record is a representation of some physical or conceptual object. Say, for
example, that you want to keep track of a business’s customers. You assign a
record for each customer. Each record has multiple attributes, such as name,
address, and telephone number. Individual names, addresses, and so on are
the data.
A database consists of both data and metadata. Metadata is the data that
describes the data’s structure within a database. If you know how your
data is arranged, then you can retrieve it. Because the database contains a
description of its own structure, it’s self-describing. The database is integrated
because it includes not only data items but also the relationships among data
items.
The database stores metadata in an area called the data dictionary, which
describes the tables, columns, indexes, constraints, and other items that
make up the database.
Because a flat file system (described later in this chapter) has no metadata,
applications written to work with flat files must contain the equivalent of the
metadata as part of the application program.
Database Size and Complexity
Databases come in all sizes, from simple collections of a few records to
mammoth systems holding millions of records.
A personal database is designed for use by a single person on a single computer.
Such a database usually has a rather simple structure and a relatively small
size. A departmental or workgroup database is used by the members of a single
department or workgroup within an organization. This type of database is
generally larger than a personal database and is necessarily more complex;
such a database must handle multiple users trying to access the same data at
the same time. An enterprise database can be huge. Enterprise databases may
model the critical information flow of entire large organizations.