Avoiding Pitfalls in Data Modeling - High-Performance Web Databases: Design, Development, and Deployment - page 288

Exhibit 18-15. Cardinality convention.
might need clarification or more precise definitions of the users' require-
ments. The most common combination of maximum cardinality is one-to-
many; a less common combination is many-to-many; and the least common
combination is one-to-one. The use of the second combination requires
resolution by way of an associative entity, and the use of the last one may
require additional clarification or refinement.
There are specific areas in which cardinality should be reviewed. For
example, the most generalized cardinality pair is the use of zero optionality
on both ends of a many-to-many relationship. This trait falls into a one-size-
fits-all pattern. This type of cardinality might require more specific details
(see Exhibit 15a).
On the other extreme of the spectrum, the use of mandatory optionality
on one-to-one relationships should also prompt more investigation. This
pattern represents the most definitive and narrow cardinality. (see Exhibit
15b). It even appears questionable whether the two entities involved are
part of the same entity or not.