Data Warehouse Concepts - Data Warehouse Systems: Design and Implementation - page 53

Köln

Berlin

Lyon

Paris

Q1 measure

values

Q2

dimensions

Q3

Q4

Produce Seafood

Beverages Condiments

Product (Category)

Fig. 3.1 A three-dimensional cube for sales data with dimensions Product , Time ,
and Customer ,andameasure Quantity

thedatacubeinFig. 3.1 , based on a portion of the Northwind database. We
can use this cube to analyze sales figures. The cube has three dimensions:
Product , Time ,and Customer .A dimension level represents the granularity,
or level of detail, at which measures are represented for each dimension of
the cube. In the example, sales figures are aggregated to the levels Category ,
Quarter ,and City , respectively. Instances of a dimension are called members .
For example, Seafood and Beverages are members of the Product dimension
at the Category level. Dimensions also have associated attributes describing
them. For example, the Product dimension could contain attributes such as
ProductNumber and UnitPrice , which are not shown in the figure.
On the other hand, the cells of a data cube, or facts , have associated
numeric values (we will see later that this is not always the case), called
measures . These measures are used to evaluate quantitatively various
aspects of the analysis at hand. For example, each number shown in a cell
of the data cube in Fig. 3.1 represents a measure Quantity , indicating the
number of units sold (in thousands) by category, quarter, and customer's
city. A data cube typically contains several measures. For example, another
measure, not shown in the figure, could be Amount , indicating the total sales
amount.
Adatacubemaybe sparse or dense depending on whether it has
measures associated with each combination of dimension values. In the case
of Fig. 3.1 , this depends on whether all products are bought by all customers
during the period of time considered. For example, not all customers may have
ordered products of all categories during all quarters of the year. Actually, in
real-world applications, cubes are typically sparse.