My first Publication Agile-Data-Warehouse-Design-eBook | Page 72

Modeling Business Events
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Figure 2-10
Adding quantities
Unit of Measure
When you ask the stakeholders for example quantity values, you should also
discover their unit of measure. If you find that a quantity is captured in multiple
units of measure it will need to be stored in a standard unit in the data warehouse
to produce useful additive facts, so you should ask the stakeholders what that
standard unit should be. (Chapter 8 provides details on designing additive facts.)
You record the unit of measure in the quantity’s column type using square brackets
type notation; e.g., [$] or [Kg]. The unit of measure is a more useful descriptive
type for a quantity than [how many].
Ask for the standard
unit of measure for
each quantity
[ ]: Square brackets denote detail type (e.g. who, where) and unit of measure for
how many details.
If a quantity needs to be reported in multiple units of measure you can record them
as a list with the standard unit of measure first. Figure 2-10 shows examples events
where REVENUE is captured in dollars and pounds. The column type [$, £, €, ¥]
records that US Dollar is the standard unit for the data warehouse, but BI applica-
tions will also need to report REVENUE in Sterling, Euro and Yen.
Durations
You discover durations by asking how long questions. For example, “How long
does it take a CUSTOMER to order a PRODUCT?” If the stakeholders view the
event as a point in time there will be no duration (not recorded or not significant).
Asking for durations is another way of testing if the event should be modeled as
evolving. Duration calculations can expose missing when details and highlight
other events (verbs) that are so closely related to the current event that they should
all be part of an evolving event.
Multiple units of
measure can be
listed in the column
type
Ask “How long?” to
discover durations
and evolving events