Database Reference
In-Depth Information
Figure 3-8.
Complex Network
3.5.3 Multiplicity of Relationships
It is customary to indicate on the ERD (or ORD), the
multiplicity
(also called the
cardinality
) of each relationship. By this we mean, how many occurrences of one entity
(or object type) can be associated with one occurrence of the other entity (or object type).
This information is particularly useful when the system is being constructed. Moreover,
violation of multiplicity constraints could put the integrity of the system is question,
which of course is undesirable. Usually, the DBMS does not facilitate enforcement of
multiplicity constraints at the database level. Rather, they are typically enforced at the
application level by the software engineer.
Several notations for multiplicity have been proposed, but the Chen notation (first
published in 1976, and reiterated in [Chen, 1994]) is particularly clear; it is paraphrased
here: Place beside each entity (or object type), two numbers [
x
,
y
]. The first number (
x
)
indicates the minimum participation, while the second (
y
) indicates the maximum
participation.
An alternate notation is to use two additional symbols along with the Crow's Foot
notation: an open circle to indicate a participation of zero, and a stroke (|) to indicate a
participation of 1. The maximum participation is always indicated nearest to the entity (or
object type) box.
For convenience, you could also use the Chen's notation for multiplicity, along
with the Crow's Foot notation for representing the relationships. The Chen notation
is preferred because of its clarity and the amount of information it conveys. Figure
3-9
provides an illustrative comparison of the two notations.
