Databases Reference
In-Depth Information
Figure 12: A mandatory n:1 association in (a) UML, (b) IE, (c) Barker ER, and (d) ORM
Figure 13: Some mandatory and frequency constraints with no graphic equivalent in IE,
Barker ER or UML
nor can it capture a minimum occurrence frequency above 1 for any sequence of fewer
than n−1 roles. This is because multiplicity on one role is defi ned in terms of the other n−1
roles. This is fi ne for binary associations, where n−1 = 1, but not for ternaries and beyond.
For example, none of the mandatory role or frequency constraints expressed in Figure 13
can be graphically expressed in UML (or IE or Barker ER, for that matter). For practical
examples of such constraint patterns, see Halpin (2001b). This weakness stems from plac-
ing multiplicities on a “far” end of an association rather than directly on the determining
roles, and confl ating global and local aspects in the same concept. Considerable care is
required in choosing constraint primitives if the modeling notation is to scale properly to
n-ary associations.
