THE DATABASE MANAGEMENT SYSTEM CONCEPT - Fundamentals of Database Management Systems - page 46

3.1 E NTITIES AND A TTRIBUTES

YOUR

TURN

E ntities and their attributes are all
around us in our everyday lives. Normally, we don't stop
to think about the objects or events in our world formally
as entities with their attributes, but they're there.

the chosen entity's attributes. Then, generalize them to
''type.'' For example, you may have a backpack (an
entity) that is green in color (an attribute of that entity).
Generalize that to the entity set of all backpacks and
to the attribute type color. Next, go through the same
exercise for an event in your life, such as taking a
particular exam, your last birthday party, eating dinner
last night, etc.

Q UESTION :
Choose an object in your world that you interact with
frequently. It might be a university, a person, an
automobile, your home, etc. Make a list of some of

Basic Concepts in Storing and Retrieving Data
Having established the idea of a file and its records, we can now, in simple terms at
this point, envision a company's data as a large collection of files. The next step is to
discuss how we might want to access data from these files and otherwise manipulate
thedatainthem.

Retrieving and Manipulating Data There are four fundamental operations that can
be performed on stored data, whether it is stored in the form of a simple linear file,
such as that of Figure 3.2, or in any other form. They are:

Retrieve or Read ■

Insert

■

Delete

■

Update ■

It is convenient to think of each of these operations as basically involving one
record at a time, although in practice they can involve several records at once, as
we will see later in the topic. Retrieving or reading a record means looking at a
record's contents without changing them. For example, using the Salesperson file
of Figure 3.2, we might read the record for salesperson 204 because we want to
find out what year she was hired. Insertion means adding a new record to the file,
as when a new salesperson is hired. Deletion means deleting a record from the
file, as when a salesperson leaves the company. Updating means changing one or
more of a record's field values, for example if we want to increase salesperson
420's commission percentage from 10 to 15. There is clearly a distinction between
retrieving or reading data and the other three operations. Retrieving data allows a
user to refer to the data for some business purpose without changing it . All of the
other three operations involve changing the data. Different topics in this topic will
focus on one or another of these operations simply because a particular one of the
four operations may be more important for a particular topic than the others.
One particularly important concept concerning data retrieval is that, while
information systems applications come in a countless number of variations, there
are fundamentally only two kinds of access to stored data that any of them require.