Other issues arise when considering items such as currencies. In a global business,
there are often source systems in each country recording receivables, and normally
the business systems store these receivables in the local currency. If a business has
branches in London, Paris, Tokyo, and New York, it??™s likely that data is stored in
Pounds, Euros, Yen, and US Dollars. In building the warehouse, sales measured in
Yen cannot simply be added to sales in US Dollars in order to get non-European
sales. Instead, the data must be transformed by applying some form of currency
conversion to the data before storing it in the warehouse. Whether this is done by
44 B u s i n e s s I n t e l l i g e n c e w i t h M i c r o s o f t O f f i c e P e r f o r m a n c e P o i n t S e r v e r 2 0 0 7
using a day-to-day exchange rate table, the average exchange rate over a period
of time, or any other method, is strictly up to the business. Therefore, business
requirements often come into play during the design phase of the ETL process.
While this section on ETL will be relatively brief, there is one point that is
absolutely essential: ETL often consumes the majority of the effort in building a
data mart or warehouse. Some projects spend up to 80 percent of the total time
in the ETL phase. This is due to a number of factors, including the complexity of
identifying and extracting back end data, applying complex transformations to data
to meet business rules, handling data for slowly changing dimensions, and working
with dirty data.
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