The Data Model Resource Book, Volume 3: Universal Patterns for Data Modeling

Silverston c07.tex V2 - 11/20/2008 5:44pm Page 323

Level 1 Contact Mechanism Pattern 323

This pattern does not accommodate relating contact mechanisms to

other entities (or to itself recursively). Because this pattern does not

maintain contact mechanism data in its own entity, it does not allow

relationships from contact mechanisms to other entities. For example,

the same contact mechanism, such as a postal address, may be related to

a PARTY as well as to an ORDER, a SHIPMENT, an INVOICE, and other

entities. Another example is that contact mechanisms may be linked to

other contact mechanisms; for example, a person’s fax may be linked

to his email so that when he gets a fax it automatically sends him

an email.

This pattern does not handle multiplicity of attributes very well. If a

person has ten telephone numbers, you should have at least ten different

attributes for a PARTY. If each of these telephone numbers has two dif-

ferent purposes, you would have twenty attributes for the entity! S o you

can see that attributes can multiply rather easily with this pattern. There-

fore, when you have a lot of the same type of contact mechanisms for an

entity, using this pattern has great disadvantages. For example, when

there are many telephone numbers for a PARTY, use of this pattern is

violating the first normal form of normalization (repeating groups), and

this could cause more and more attributes to be