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and dynamic hierarchies; multiple and optional arcs; and additivity. Though some
of them are not necessary in the simplest and most common modeling situations,
they are quite useful in order to better express the multitude of conceptual shades
that characterize real-world scenarios. In particular we will see how, following the
introduction of some of this constructs, hierarchies will no longer be defined as trees
to become, in the general case, directed graphs.
Descriptive Attributes
In several cases it is useful to represent additional information about a dimension
attribute, though it is not interesting to use such information for aggregation. For
instance, the user may ask for knowing the address of each store, but the user will
hardly be interested in aggregating sales according to the address of the store.
Definition 8: A descriptive attribute specifies a property of a dimension
attribute, to which is related by an x-to-one association. Descriptive attributes
are not used for aggregation; they are always leaves of their hierarchy
and are graphically represented by horizontal lines.
There are two main reasons why a descriptive attribute should not be used for aggregation:
Guideline. 5: A descriptive attribute either has a continuously-valued
domain (for instance, the weight of a product), or is related to a dimension
attribute by a one-to-one association (for instance, the address of a
customer).
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