Databases Reference
In-Depth Information
Minimum
cardinality for the
Rep entity (1: there must be
at least one sales rep
for a customer)
Maximum cardinality
for the Customer entity
(many: a sales rep can have
many customers)
Rep
Represents
Customer
218
Symbol for
”optional”(minimum
cardinality for the Customer
table is 0: a rep can have
zero customers)
Maximum cardinality
for the Rep entity (1: there
must be at most one sales
rep for a customer)
FIGURE 6-43
E-R diagram that represents cardinality
The crow's foot to left of the Customer rectangle indicates that the maximum cardinality is “many.” The
circle to the left of the crow's foot indicates that the minimum cardinality is zero; that is, a sales rep could be
associated with zero customers. An entity in a relationship with minimum cardinality of zero plays an
optional
role
in the relationship. An entity with a minimum cardinality of one plays a
mandatory role
in the
relationship.
