Database 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
222
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.
'
