Database Reference
In-Depth Information
Dicing
is supported by QB. For instance, the
FILTER
clause in SPARQL
can be used to implement a dicing condition.
Given the limitations commented above, in order to be able to define
and implement OLAP operations, we must extend QB to support dimension
hierarchies. We next explain how this can be done.
14.3.2 QB4OLAP Vocabulary
The
QB4OLAP vocabulary
14
aims at giving a solution to the problems
of the QB vocabulary discussed in the previous section. Multidimensional
data can be published in QB4OLAP from scratch, or data already published
using QB can be extended with dimension levels, level members, dimension
hierarchies, and the association of aggregate functions to measures without
affecting the existing observations. Therefore, data cubes already published
using QB can also be represented using QB4OLAP without affecting existing
applications developed over QB data cubes.
Figure
14.7
depicts the QB4OLAP vocabulary. Classes and proper-
ties added to QB (with prefix
qb4o
) are depicted with light gray back-
ground and black font. The class
qb4o:LevelProperty
models dimension
levels. Relations between dimension levels are represented using the property
qb4o:parentLevel
. Level members are represented as instances of the class
qb4o:LevelMember
, and relations between them can be expressed using the
property
skos:broader
. Level attributes are defined via the
qb4o:hasAttribute
property. Level properties are stated in the data structure definition. The
class
qb4o:AggregateFunction
represents aggregate functions. The association
between measures and aggregate functions is represented using the property
qb4o:hasAggregateFunction
. This property, together with the concept of
component sets, allows a given measure to be associated with different
aggregate functions in different cubes.
Multidimensional Data Representation in QB4OLAP
We next show how QB4OLAP can be used to publish multidimensional data
from scratch. The definition of the schema of the
Geography
dimension using
QB4OLAP is shown next:
ex:Geography a qb:DimensionProperty .
ex:UnitaryAuthority a qb4o:LevelProperty ; qb4o:inDimension ex:Geography ;
qb4o:parentLevel ex:GovernmentO
ceRegion .
ex:GovernmentO
ceRegion a qb4o:LevelProperty ; qb4o:inDimension ex:Geography ;
qb4o:parentLevel ex:Country .
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