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Malinowski and Zimányi (2008); these elements
exist in real-world applications and are required
during the decision-making process. Many of
these hierarchies and dimensions can already be
implemented in commercial tools, e.g., in Mi-
crosoft SQL Server Analysis Services (SSAS).
However, these hierarchies and dimensions cannot
be distinguished either at the logical level (i.e.,
star or snowflake schemas) or in the OLAP cube
designer. We will demonstrate the importance of
using a conceptual model, such as the MultiDim
model (Malinowski & Zimányi, 2008) to facilitate
the process of understanding user requirements by
characterizing different kinds of hierarchies and
dimensions. By using the MultiDim model as an
example of a conceptual model we do not intent
to suggest that this is the only model that responds
to analytical needs. On the contrary, we leave to
the designers the decision of using a conceptual
multidimensional model among several other al-
ready existing models (e.g., A. Abelló, Samos &
Saltor, 2006; Luján-Mora, Trujillo & Song, 2006;
Rizzi, 2007) that may better fit their needs.
We have chosen SSAS as an example of a
commercial OLAP tool, because to our knowl-
edge, it provides different kinds of hierarchies
and dimensions that can be included in the cube
without incurring into any programming effort,
i.e., using a wizard or the click-and-drag mecha-
nism. We will compare the representation of dif-
ferent kinds of hierarchies and dimensions in the
MultiDim model, in the relational logical model
(star or snowflake schema), and in the SSAS
OLAP cube designer.
This chapter does not focus on details as de-
scribed in Malinowski and Zimányi (2008) for
representing different kinds of hierarchies and
dimensions. The main objective is to show how
many concepts already implemented in commer-
cial tools and accepted by the practitioners may
be better understood and correctly specified if the
practice for the DW design changes and includes
a representation at the conceptual level.
In the next section we survey works related
to conceptual modeling for DW and OLAP ap-
plications. Then, after introducing a motivating
example that is used throughout this chapter, we
present the main features of the MultiDim model.
The following two sections refer to the conceptual
representation and implementation of different
kinds of hierarchies and dimensions. The last
section includes conclusions.
relAted Work
The advantages of conceptual modeling for data-
base design have been acknowledged for several
decades and have been studied in many publica-
tions. However, the analysis presented in Rizzi
(2003) shows the limited interest of the research
community in conceptual multidimensional mod-
eling. A detailed description of multidimensional
models can be found in Rafanelli (2003) and
Torlone (2003) and a comparison of proposals
for different kinds of hierarchies in Malinowski
and Zimányi (2008).
Current multidimensional models support
many hierarchies (e.g., Luján-Mora et al., 2006;
Rizzi, 2007; Sapia et al., 1998; Tryfona et al.,
1999), in some cases, using different names for
the same kind of hierarchy. Some models (e.g.,
Torlone, 2003) only refer to simple hierarchies.
This situation is considered as a shortcoming of
existing models for DWs (Hümmer, Lehner, Bauer,
& Schlesinger, 2002).
Some proposals provide graphical representa-
tions based on the ER model (e.g., Sapia, Blaschka,
Höfling & Dinter, 1998; Tryfona, Buisborg &
Borch, 1999), on UML (e.g., Abelló, Samos &
Saltor, 2006; Luján-Mora et al., 2006), or on spe-
cific new notations (e.g., Rizzi, 2007; Hüsemman,
Lechtenbörger & Vossen, 2000; Tsois, Karayan-
nidis & Sellis, 2001). Other models give only a
description and/or a definition of some of these
hierarchies without a graphical representation (e.g.,
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