Database Reference
In-Depth Information
The union operation is also used to display different granularities on the
same dimension. For example, if SalesCountry is the cube in Fig. 3.4 b, then
the following operation
UNION(Sales, SalesCountry)
results in a cube containing sales measures summarized by city and by
country.
The difference operation removes the cells in a cube that exist in another
one. Obviously, the two cubes must have the same schema. For example, if
TopTwoSales is the cube in Fig. 3.4 m, then the following operation
DIFFERENCE(Sales, TopTwoSales)
will result in the cube in Fig. 3.4 t, which contains all sales measures except
for the top two sales by quarter and city.
Finally, the drill-through operation allows one to move from data at the
bottom level in a cube to data in the operational systems from which the cube
was derived. This could be used, for example, if one were trying to determine
the reason for outlier values in a data cube.
Table 3.1 summarizes the OLAP operations we have presented in this
section. In addition to the basic operations described above, OLAP tools
provide a great variety of mathematical, statistical, and financial operations
for computing ratios, variances, interest, depreciation, currency conversions,
etc.
3.3 Data Warehouses
A data warehouse is a repository of integrated data obtained from several
sources for the specific purpose of multidimensional data analysis. More
technically, a data warehouse is defined as a collection of subject-oriented,
integrated, nonvolatile, and time-varying data to support management
decisions. We explain next these characteristics:
￿ Subject oriented means that data warehouses focus on the analytical
needs of different areas of an organization. These areas vary depending
on the kind of activities performed by the organization. For example, in
the case of a retail company, the analysis may focus on product sales
or inventory management. In operational databases, on the contrary, the
focus is on specific functions that applications must perform, for example,
registering sales of products or inventory replenishment.
￿ Integrated means that data obtained from several operational and
external systems must be joined together, which implies solving problems
due to differences in data definition and content, such as differences
in data format and data codification, synonyms (fields with different
Search WWH ::




Custom Search