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ChaPter 2 • underStanding and modeling organizational SyStemS 29

Figure 2.11
Patron Improving the E-R diagram by
adding an associative entry called
RESERVATION.

is in
the
name makes
of

I’ve added
an associative

Julie
Reservation entity.

is for
has
a

Concert/Show

At first glance, we see that the PATRON gets a reservation for a CONCERT/SHOW, and the
CONCERT/SHOW can be said to have made a booking for a PATRON.
The process isn’t that simple, of course, and the E-R diagram need not be that simple either.
The PATRON actually makes a RESERVATION, as shown in Figure 2.11. The RESERVATION
is for a CONCERT/SHOW. The CONCERT/SHOW holds the RESERVATION, and the
RESERVATION is in the name of the PATRON. We added an associative entity here because a
RESERVATION was created due to the information system required to relate the PATRON and
the CONCERT/SHOW.
Again this process is quite simple, but because concerts and shows have many perfor-
mances, the E-R diagram is drawn once more in Figure 2.12. Here we add an attributive entity
to handle the many performances of the CONCERT/SHOW. In this case the RESERVATION is
made for a particular PERFORMANCE, and the PERFORMANCE is one of many that belong
to a specific CONCERT/SHOW. In turn the CONCERT/SHOW has many performances, and
one PERFORMANCE has a RESERVATION that is in the name of a particular PATRON.
To the right of this E-R diagram is a set of data attributes that make up each of the entities.
Some entities may have attributes in common. The attributes that are underlined can be searched
for. The attributes are referred to as keys and are discussed in Chapter 13.
Systems designers often use E-R diagrams to help model a file or database. It is even more
important, however, that a systems analyst understand early both the entities and relationships
in the organizational system. In sketching out some basic E-R diagrams, the analyst needs to:
1. List the entities in the organization to gain a better understanding of the organization.
2. Choose key entities to narrow the scope of the problem to a manageable and meaningful
dimension.
3. Identify what the primary entity should be.
4. Confirm the results of steps 1 through 3 through other data-gathering methods (investiga-
tion, interviewing, administering questionnaires, observation, and prototyping), as dis-
cussed in Chapters 4 through 6.