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Figure 12. Alternative hierarchies implemented as parallel hierarchies.
values, they should define these hierarchies as
parallel instead of alternative.
Notice the difference between generalized and
alternative hierarchies. Even though both kinds of
hierarchies include several paths and one analysis
criterion, the members use exclusive paths in
generalized hierarchies and participate in all paths
in alternative hierarchies. This distinction is clear
in conceptual schemas and cannot be expressed
in logical schemas.
SSAS allows the designer to define this kind of
hierarchy as shown in Figure 13a. The composed
hierarchies can be combined during the analysis
process and answer the queries that refer to both
hierarchies, e.g., “what are the sales figures for
products that belong to the bike category and are
black” as can be seen in Figure 13b.
Notice that alternative and parallel hierar-
chies are not differentiated at the logical level
even though conceptually they are different: the
composing paths in multiple hierarchies should
not be combined, while this combination can be
performed for parallel hierarchies.
Parallel Hierarchies
Parallel hierarchies
represent the situation where
a dimension has associated several hierarchies
accounting for different analysis criteria, e.g.,
the
Product
dimension in Figure 3 with
Product
colors
and
Product groups
hierarchies as well as
the
Employee
dimension with the
Assignation
and
Supervision
hierarchies. Such hierarchies
can be independent where composed hierarchies
do not share levels or dependent, otherwise.
The composing hierarchies may be of different
kinds. For example, in Figure 3 both hierarchies
of the
Product
dimension are balanced, while
the
Employee
dimension includes non-strict and
recursive hierarchies.
Parallel hierarchies are widely used. They are
represented in the logical model either as separate
tables for each level (the tables
DimProduct
,
Dim-
ProductSubcategory
, and
DimProductCategory
in
Figure 2 for the
Product groups
hierarchy in Figure
3,) or as attributes of the table corresponding to the
leaf level (the
Color
attribute in the
DimProduct
table in Figure 2 for the
Product colors
hierarchy
in Figure 3).
dIfferent kIndS of dIMenSIonS:
theIr rePreSentAtIon
And IMPleMentAtIon
The design of DWs mainly focuses on discovering
and implementing different kinds of hierarchies
since they are important in analytical processing.
However, as development of DWs increases and
expands to different areas of human activities,
the scientific community as well as practitioners
face new challenges related to representation
and implementation of different kinds of dimen-
sions.
role-Playing dimensions
In some situations the same dimension must be
used to represent its members playing different
roles (Lachev, 2005; Luján-Mora et al., 2006;
Malinowski & Zimányi, 2008). For example, a
product can be sent to some clients, while other
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