A fact is a record of an event or state of affairs, designed
to store information so that it can be viewed from a multitude of
perspectives. Its structure makes it usable in answering a virtually
unbounded range of questions you have about your business.
In multidimensional modeling (MDM),
a fact is the smallest possible unit of information. It consists
of two parts: the base facts, which are numbers or text you
will be storing and using for the analysis, and the fact header,
which is a series of elements that indicate how the base
facts relate to the real world. Facts are implemented as rows in
a relational database.
For example, I might collect information about weather patterns
in the United States and store the data in fact form. If I did,
the table might look like this:
Each fact in the table has the same structure as every other fact.
Each contains a header with two elements in it–a CITY and
a DAY–and one base fact, the maximum temperature (MAX_TEMP)
in the city for that day.
You can think of a fact as telling a story. All the facts in the
same table will tell the same kind of story, but the details for
each will be different. For the first two rows shown the stories
"The maximum temperature in Phoenix
on June 28, 1999 was 105 degrees."
"The maximum temperature in San Francisco
on June 28, 1999 was 83 degrees."
Assume for a moment that this table contains information from
every city in the world for every single day in the last year. What
kinds of analysis can I do with this table? Not much yet. First,
you need a multidimensional model.
Next topic: "What
is multidimensional modeling?"