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1.3 What Kinds of Data Can Be Mined? 9

1.3.1 Database Data
A database system, also called a database management system (DBMS), consists of a
collection of interrelated data, known as a database, and a set of software programs to
manage and access the data. The software programs provide mechanisms for defining
database structures and data storage; for specifying and managing concurrent, shared,
or distributed data access; and for ensuring consistency and security of the information
stored despite system crashes or attempts at unauthorized access.
A relational database is a collection of tables, each of which is assigned a unique
name. Each table consists of a set of attributes (columns or fields) and usually stores
a large set of tuples (records or rows). Each tuple in a relational table represents an
object identified by a unique key and described by a set of attribute values. A semantic
data model, such as an entity-relationship (ER) data model, is often constructed for
relational databases. An ER data model represents the database as a set of entities and
their relationships.

Example 1.2 A relational database for AllElectronics. The fictitious AllElectronics store is used to
illustrate concepts throughout this book. The company is described by the following
relation tables: customer, item, employee, and branch. The headers of the tables described
here are shown in Figure 1.5. (A header is also called the schema of a relation.)

The relation customer consists of a set of attributes describing the customer infor-
mation, including a unique customer identity number (cust ID), customer name,
address, age, occupation, annual income, credit information, and category.
Similarly, each of the relations item, employee, and branch consists of a set of attri-
butes describing the properties of these entities.
Tables can also be used to represent the relationships between or among multiple
entities. In our example, these include purchases (customer purchases items, creating
a sales transaction handled by an employee), items sold (lists items sold in a given
transaction), and works at (employee works at a branch of AllElectronics).

customer (cust ID, name, address, age, occupation, annual income, credit information,
category, ...)
item (item ID, brand, category, type, price, place made, supplier, cost, ...)
employee (empl ID, name, category, group, salary, commission, ...)
branch (branch ID, name, address, ...)
purchases (trans ID, cust ID, empl ID, date, time, method paid, amount)
items sold (trans ID, item ID, qty)
works at (empl ID, branch ID)
Figure 1.5 Relational schema for a relational database, AllElectronics.