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

Silverston c04.tex V2 - 11/21/2008 3:02am Page 155

Level 2 Expanded Recursive Pattern 155

have added subtypes to WORK EFFORT with the specific relationships and

attributes. However, the generalized recursive relationship will not support

specific optionality between different levels of a hierarchy (or aggregation):

for example, if a PROJECT must have one or more PHASE(s), but a PHASE

may have one or more TASK(s).

The pattern is more abstract than the level 1 pattern, but it should be

fairly easy to explain to technical audiences. In general, more documentation

is needed in the data model, such as showing instance diagrams, worked

examples, and/or spreadsheets and databases with e xample data, because the

model is not as self-explanatory as the level 1 pattern.

Another key limitation of this pattern is that it provides only one-to-many

relationships. Therefore, if there is a hierarchy where there is need for a child

to be related to more than one parent, this pattern does not support this need.

This may be referred to as either a ‘‘many-to-many’’ or a ‘‘matrix’’ relationship

structure and is covered in the next section.

Level 2 Expanded Recursive Pattern

Most enterprises need flexibility when dealing with hierarchies, aggregations,

or peer-to-peer relationships. The Level 2 Recursive Pattern supports this

flexibility, but with some loss of specific understanding. This is the classic

balance of flexibility ...