Logical Data Warehouse Design - Data Warehouse Systems: Design and Implementation - page 151

detailed discussion on this topic. In Analysis Services, there are two types
of hierarchies. Attribute hierarchies correspond to a single column in a
dimension table, for instance, attribute ProductName in dimension Product .
On the other hand, multilevel (or user-defined ) hierarchies are derived
from two or more attributes, each attribute being a level in the hierarchy,
for instance, Product and Category . An attribute can participate in more
than one multilevel hierarchy, for instance, a hierarchy Product and Brand
in addition to the previous one. Analysis Services supports three types of
multilevel hierarchies, depending on how the members of the hierarchy are
related to each other: balanced, ragged, and parent-child hierarchies. We
will explain how to define these hierarchies in Analysis Services later in this
section.
We illustrate next how to define the different kinds of dimensions
supported by Analysis Services using the Northwind cube. We start with
a regular dimension ,namely,the Product dimension, shown in Fig. 5.22 .
The right pane defines the tables in the DSV from which the dimension is
created. The attributes of the dimension are given in the left pane. Finally,
the hierarchy Categories ,composedofthe Category and Product levels, is
shown in the central pane. The attributes CategoryKey and ProductKey are
used for defining these levels. However, in order to show friendly names when
browsing the hierarchy, the NameColumn property of these attributes are set
to CategoryName and ProductName , respectively.
Fig. 5.22 Definition of the Product dimension
Figure 5.23 shows some members of the Product dimension. As shown
in the figure, the names of products and categories are displayed in the
dimension browser. Notice that a member called Unknown is shown at the
bottom of the figure. In fact, every dimension has an Unknown member. If
a key error is encountered while processing a fact table, which means that a
corresponding key cannot be found in the dimension, the fact value can be
assigned to the Unknown member for that dimension. The Unknown member
can be made visible or hidden using the dimension property UnknownMember .
When set to be visible, the member is included in the results of the MDX
queries.