Data Modeling with the Entity-Relationship Model - Database Processing: Fundamentals, Design, and Implementation - page 187

DEPARTMENT 1:N EMPLOYEE

(a) Original E-R Model Version

DEPARTMENT EMPLOYEE

Figure 5-7
Two Versions of a 1:N
O-M Relationship

(b) Crow's Foot Version

its own symbols while putting an object-oriented programming spin on it. UML notation is
summarized in Appendix D.

By ThE Way In addition to differences due to different versions of the E-R model, there
also are differences due to software products. For example, two products
that both implement the IE Crow's Foot model may do so in different ways. The result is a
mess. When creating a data model diagram, you need to know not just the version of the
E-R model you are using, but also the idiosyncrasies of the data modeling product you use.

E-R Diagrams Using the IE Crow's Foot Model
Figure 5-7 shows two versions of a one-to-many, optional-to-mandatory relationship. Figure 5-7(a)
shows the original E-R model version. Figure 5-7(b) shows the crow's foot model using common
crow's foot symbols. Notice that the relationship is drawn as a dashed line. The reason for this will
be explained later in this chapter. For now, notice the crow's foot symbol used to show the many
side of the relationship.
The crow's foot model uses the notation shown in Figure 5-8 to indicate the relationship
cardinality. The symbol closest to the entity shows the maximum cardinality, and the other sym-
bol shows the minimum cardinality. A hash mark indicates one (and therefore also mandatory),

Figure 5-8
Crow's Foot Notation

Symbol Meaning Numeric Meaning

Mandatory—One Exactly one

Mandatory—Many One or more

Optional—One Zero or one

Optional—Many Zero or more