As
an example, the previous cube view can be denoted simply as CV[Department].
8 Hurtado & Gutierrez
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Rollup and Drilldown Operations
There are two main operators to move among the different cube views in a data
cube, namely the rollup and drilldown operators (Jarke, Lenzerini, Vassiliou, &
Vassiliadis, 1997). Given two granularities G and G??™, a rollup operation aggregates
the cube view CV[G] to the granularity G??™. As an example, a rollup from a singlecategory
granularity Category to the granularity Department in a data cube for the
dimension of Figure 1 can be defined using the following aggregate query:
SELECT Department, SUM(Amount)
FROM CV[Category],?“[Category,Department]
WHERE CV[Category].Category=?“[Category,Department].Category
GROUP BY Department.
This rollup operation, applied to the cube view CV[Category] aggregates the facts
in it to the granularity of aggregation specified by the category Department. In the
context of a fixed data cube we abbreviate this rollup operation as ROLLUP Category
TO Department. Rollup operations have also been called summarizations
(Lenz & Shoshani, 1997) or consolidations (Harinarayan, Rajaraman, & Ullman,
1996).
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