this pdf excerpt

nbina-
These
_rs repployee
s con-
,1 COn-
binary
entity
) type,
-tment
'ee can
ios for
rlates a
:esents
lanage
lationminin
have
'3u
EMPLOYEE
€1
e2
€a
3.4 Relationship Types, Relationship Sets, Roles, and Structural Constraints
WORKS ON PROJECT
t-ardinality ratios for binary relationships are represented on ER diagrams by displaving
l, M, and N on the diamonds as shown in Figure 3.2.
Participation Constraints and Existence Dependencies. The participation
constraint specifies whether the exister.rce of an entity depends on its being reiated
ttr irnother entity via the relationship type. This constraint specifies the minimurn
number of relationship instances that each entity can participate in, and is somet
imes called the minimum cardinality constraint. There are two types of participation
constraints-total and partial-which we illustrate by example. If a company
policy states thal every employee must work for a department, then an employee
r.rltity can exist only if it participates in at least one WORKS FOR relationship
instance (Figure 3.9). Thus, the participation of EMPLOYEE in WORKS_FOR is
c.rlled total participation, meaning that every entity in the total sel of employee
cntities must be related to a department entity via WORKS_FOR. Total participation
is trlso called existence dependenry. In Figure 3.12 we do not expect every employee
to nanage a department, so the participation of EMPLOYEE in the MANAGES reiationship
type is partial, meaning thar some or part of the set o/employee entities are
rc'lated to some department entity via MANAGES, but not necessarily all. We will
ret-er to the cardinality ratio and participation constraints, taken together, as the
structural constraints of a relationship type.
Pt
Pz
Ps
P4
Figure 3.13
An M:N relationship,
WORKS_ON