Database Reference
In-Depth Information
level member. For instance, it is stated that the member
ns1:J
belongs
to level
ex:GovernmentO
ceRegion
and that such member aggregates over
the member
ns2:921
in level
ex:Country
. Note that the dimension instances
are treated analogously to QB, using the SKOS vocabulary.
ThedatacubeofFig.
14.5
can be represented using QB4OLAP as follows:
ex:Geography a qb:DimensionProperty .
ex:Time a qb:DimensionProperty .
ex:Year a qb4o:LevelProperty ; skos:closeMatch db:Year ; qb4o:inDimension ex:Time .
ex:Household a qb:MeasureProperty .
ex:HouseholdCS a qb:DataStructureDefinition ;
qb:component [qb4o:level ex:UnitaryAuthority] ;
qb:component [qb4o:level ex:Year] ;
qb:component [qb:measure ex:Household ; qb4o:hasAggregateFunction qb4o:sum] .
There are some relevant differences with the QB data cube shown in
the previous section. The data cube schema is declared as an instance
of the class
qb:DataStructureDefinition
. The level property
qb4o:level
is
used to specify the levels in the cube schema. For example, the triple
ex:HouseholdCS qb:component [qb4o:level ex:Year]
tells that
ex:Year
is a
level of the cube. Analogously, the triple
ex:Household a qb:MeasureProperty
tells that
ex:Household
is a measure in the cube, like in QB. In addition,
the aggregate function corresponding to this measure is defined using the
property
qb4o:hasAggregateFunction
.
An instance of the cube is represented in QB4OLAP as follows:
ex:dataset-hh a qb:DataSet ; qb:structure ex:HouseholdCS ;
rdfs:label
''
HouseholdinUK
''
@en .
ex:o1 a qb:Observation ; qb:dataSet ex:dataset-hh ;
ex:UnitaryAuthority ns0:00mc ; ex:Year db:2007 ; ex:Household 58 .
The data set
ex:dataset-hh
is defined as an instance of the class
qb:DataSet
,
linked to the data cube using the property
qb:structure
.Further,anobserva-
tion
ex:o1
is defined as an instance of the class
qb:Observation
.Theinstance
represents the cell in the cube with values
ns0:00mc
for
UA
,
2007
for
Year
,
and
58
for the measure
Household
. Note that observations are defined using
QB classes and properties. Therefore, the observations in an existing cube
expressed in QB do not need to be changed in order to express the cube in
QB4OLAP.
OLAP Operations over QB4OLAP
We now show a possible implementation of some of the OLAP operations
over a data cube defined using the QB4OLAP vocabulary. The operations
arewritteninSPARQL.
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