Improving Expressive Power in Modeling Data Warehouse and OLAP Applications - Evolving Application Domains of Data Warehousing and Mining - page 30

Figure 9. A non-covering hierarchy: a) schema and b) example of instances

The unbalanced and non-covering hierar-
chies represent different situations and can be
clearly distinguished using a conceptual model
(respectively, the Sales territory and Location
hierarchies in Figure 3). However, SSAS considers
implementation details that are very similar for
both hierarchies, i.e., both include placeholders
or null values in missing levels and states that “it
may be impossible for end users to distinguish
between unbalanced and ragged hierarchies”;
they even consider excluding this kind of hierar-
chy from the future editions of Analysis Services
(Microsoft, 2005).
Unbalanced and non-covering hierarchies also
differ in the process of measure aggregation. For
an unbalanced hierarchy, the measure values are
repeated from the parent member to the missing

child members and cannot be aggregated during
the roll-up operations. For the non-covering hi-
erarchies, the measures should be aggregated for
every parent member represented as placeholder
or null value in order to propagate aggregation
correctly to other higher levels.

Non-Strict Hierarchies

The above-mentioned simple hierarchies only
include a parent-child relationship with many-to-
one cardinalities, i.e., a child member is related to
at most one parent member and a parent member
may be related to several child members. However,
in real-world applications different situations
may occur, e.g., an employee can work in several
departments or a mobile phone can be classified

Figure 10. A non-covering hierarchy in SSAS: a) schema and b) instances