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ChaPter 2 • underStanding and modeling organizational SyStemS 27

Figure 2.8
is assigned to
Employee Office Examples of different types of
is occupied by
relationships in E-R diagrams.

will serve
Cargo Aircraft Distribution Center
craft
is served by

is assigned to
Systems Analyst Project
will be developed by

is undergoing Scheduled
Machine
is being done to Maintenance

is assigned to
Salesperson Customer
is called on by

has
Home Office Employee
is assigned to

is flying to
Passenger Destination
will be visited by

Notice that when a many-to-one relationship is present, the grammar changes from “is” to
“are,” even though the singular “is” is written on the line. The crow’s foot and the single mark
do not literally mean that this end of the relationship must be a mandatory “many.” Instead, they
imply that this end could be anything from one to many.
Figure 2.8 elaborates on this scheme. Here we have listed a number of typical entity rela-
tionships. The first, “An EMPLOYEE is assigned to an OFFICE,” is a one-to-one relationship.
The second one is a one-to-many relationship: “One CARGO AIRCRAFT will serve one or
more DISTRIBUTION CENTERs.” The third one is slightly different because it has a circle at
one end. It can be read as “A SYSTEMS ANALYST may be assigned to MANY PROJECTs,”
meaning that the analyst can be assigned to no projects [that is what the circle (O), for zero, is
for], one, or many projects. Likewise, the circle (O) indicates that none is possible in the next
relationship. Recall that the short mark means one. Therefore, we can read it as follows: “A
MACHINE may or may not be undergoing SCHEDULED MAINTENANCE.” Notice that the
line is written as “is undergoing,” but the end marks on the line indicate that either no mainte-
nance (O) or maintenance (I) is actually going on.
The next relationship states, “One or many SALESPEOPLE (plural of SALESPERSON) are
assigned to one or more CUSTOMERs.” It is the classic many-to-many relationship. The next
relationship can be read as follows: “The HOME OFFICE can have one or many EMPLOYEEs”
or “One or more EMPLOYEEs may or may not be assigned to the HOME OFFICE.” Once
again, the I and O together imply a Boolean situation—in other words, one or zero.
The final relationship shown here can be read as, “Many PASSENGERs are flying to many
DESTINATIONs.” Some people prefer this symbol [>—+]to indicate a mandatory “many”