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28 Part 1 • SyStemS analySiS FundamentalS

Figure 2.9
Three different types of entities Fundamental Usually a real entity: a person,
used in E-R diagrams. Entity place, or thing

Associative Something created that joins
Entity two entities

Something useful in describing
Attributive
Entity attributes, especially repeating
groups

condition. (Would it ever be possible to have only one passenger or only one destination?) Even
so, some CASE tools such as Visible Analyst do not offer this possibility because the optional
one-or-many condition as shown in the SALESPERSON–CUSTOMER relationship will do.
Up to now we have modeled all our relationships using just one simple rectangle and a line.
This method works well when we are examining the relationships of real things such as real
people, places, and things. Sometimes, though, we create new items in the process of develop-
ing an information system. Some examples are invoices, receipts, files, and databases. When we
want to describe how a person relates to a receipt, for example, it becomes convenient to indicate
the receipt in a different way. There are three different types of entities: the fundamental entity,
the associative entity, and the attributive entity are depicted in Figure 2.9.
An associative entity can exist only if it is connected to at least two other entities. For that
reason, some call it a gerund, a junction, an intersection, or a concatenated entity. This wording
makes sense because a receipt wouldn’t be necessary unless there were a customer and a sales-
person making the transaction.
Another type of entity is an attributive entity. When an analyst wants to show data that are
completely dependent on the existence of a fundamental entity, an attributive entity should be
used. For example, if a library has multiple copies of the same book, an attributive entity can be
used to designate which copy of the book is being checked out. The attributive entity is useful for
showing repeating groups of data. For example, suppose we are going to model the relationships
that exist when a patron gets tickets to a concert or show. The entities seem obvious at first: “a
PATRON and a CONCERT/SHOW,” as shown in Figure 2.10. What sort of relationship exists?

Figure 2.10
The first attempt at drawing an Patron
E-R diagram.

makes Here is
attempt.
a makes a my first
booking reservation Ken
for for

Concert/Show