Avoiding Pitfalls in Data Modeling - High-Performance Web Databases: Design, Development, and Deployment - page 287

Exhibit 18-13. Time dimension affecting semantics and cardinality.

the modeler assumes certain time dimensions that differ from what the
users have in mind.

In general, time is not modeled as an object in data
modeling, but situations can arise in which time periods could be of inter-
est and significance.

Time as an Entity.

1

For example, window periods allowed for different kinds of fish that
sport fishermen can catch might necessitate the creation of an entity Time
Period, as shown in Exhibit 14. This is based on the assumption that some
special regulations are associated with each time period.

Cardinality and Optionality

Cardinality constraints represent the upper and lower bounds on the
number of instances in one entity type with respect to instances of another
type. It represents to what degree the relationship exists between two enti-
ties. The cardinality is commonly expressed as a qualifier for relationships
such as one-to-one (1-1), one-to-many (1-m) or many-to-many (m-m) rela-
tionship. Note that the common use refers to the maximum cardinality (the
upper bounds) only. Exhibit 15 illustrates the two types of cardinality,
called maximum and minimum cardinality. Cardinality in this article gener-
ally refers to maximum cardinality, and optionality generally refers to min-
imum cardinality.

Critical examination of the
cardinality of the entity-relationship model often highlights areas that

Cardinality and Telltale Signs of the Model.

Exhibit 18-14. Time period as entity type.