Dimensions as Hierarchies

VDM recognizes three types of dimensions from the perspective of their behavior as hierarchies:

Trivial Single-Level

These dimensions are usually modeled as a single entity. They carry an identifier and basic descriptive attributes, such as a name and description. They potentially may have additional attributes that can be used to aggregate facts. As an example, take a Geo-location dimension, which may have Zip-Code as an attribute. We may aggregate and report sales by zip-code and retain summaries with zip as one of the aggregate dimensions.

Fixed Level Hierarchies

Most traditional dimensions fall in this category. They are organized in a fixed number of levels, hierarchically linked according to their type. This general aspect is depicted by a general entity described by a unique identity and a few common attributes such as name and description. Individual levels of the hierarchy may have their own special attributes, usually represented as separate entities, subclasses of the general entity. An example of such a hierarchy is a Location Hierarchy, where stores comprise districts, which in turn comprise regions etc. It is not uncommon to accommodate lop-sided hierarchies by introducing pseudo-levels thus achieving a fixed depth structure

Dynamic "Tagged" Hierarchies

These hierarchies define their structure as they grow. A branch can dynamically change levels, one branch can be deeper than another, and the node types have no predefined ancestry order. Their nodes are linked into a node hierarchy and tagged according to their type. The same type may occur recursively at multiple levels in an ancestry path. Although there is no predefined static type hierarchy, certain composition rules invariably exist that determine what is allowed in the structure. These rules are observed as the hierarchy is maintained. These types of hierarchies can be mapped to a pseudo-level structure of tagged nodes, or related through nested intervals (also called nested sets.) A good example of such hierarchies are Financial Element Hierarchies or Chart of accounts. Another example may be an organizational/reporting hierarchies

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