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Figure 11-3 OLAP Metamodel: Inheritance from Object Model
Figure 11-3 illustrates how classes of the OLAP metamodel inherit from the CWM
Object Model. Two classes requiring further explanation are:
• Measure: A subclass of Attribute that describes the meaning of values stored in the data cells of a multidimensional structure.
Different OLAP models often give different interpretations to the term “measure.? In a relational Star Schema,
March 2003 OMG-CWM, v1.1: Organization of the OLAP Package
individual measures might be represented by non-key columns of a Fact table (for example, “Sales? and “Quantity? columns).
In this case, measure may be an attribute of a Cube or CubeRegion that models the Fact table. On the other hand, measures
can also be represented by members of a Measure Dimension. A Fact table supporting this representation has a single Measure
column with column values consisting of the members “Sales? and “Quantity,? and a single “value? column; that is, an implicit
data dimension storing the corresponding measure values. A similar notion of Measure Dimension is used in modeling pure hypercube
representations of multidimensional servers. Thus, the concept of measure can be represented either as a Dimension or as an
Attribute, depending on the type of OLAP system being modeled.
• Coded Level: A subclass of Level that assigns a unique encoding, or label, to each of its members. CodedLevel is not essential
to the OLAP metamodel, but is provided as a helper class for certain applications that might benefit from the ability of OLAP
systems to structure data hierarchically. For example, CodedLevel could be used to model systems of nomenclature or classification.