Organizational Understanding and Data Understanding - Data Mining for the Masses - page 18

In order to keep our transactional databases running quickly and smoothly, we may wish to create
a data warehouse. A data warehouse is a type of large database that has been denormalized and
archived. Denormalization is the process of intentionally combining some tables into a single
table in spite of the fact that this may introduce duplicate data in some columns (or in other words,
attributes).
Figure 2-3: A combination of the tables into a single data set.
Figure 2-3 depicts what our simple example data might look like if it were in a data warehouse.
When we design databases in this way, we reduce the number of joins necessary to query related
data, thereby speeding up the process of analyzing our data. Databases designed in this manner are
called OLAP (online analytical processing) systems.
Transactional systems and analytical systems have conflicting purposes when it comes to database
speed and performance. For this reason, it is difficult to design a single system which will serve
both purposes. This is why data warehouses generally contain archived data. Archived data are
data that have been copied out of a transactional database. Denormalization typically takes place at
the time data are copied out of the transactional system. It is important to keep in mind that if a
copy of the data is made in the data warehouse, the data may become out-of-synch. This happens
when a copy is made in the data warehouse and then later, a change to the original record
(observation) is made in the source database. Data mining activities performed on out-of-synch
observations may be useless, or worse, misleading. An alternative archiving method would be to
move the data out of the transactional system. This ensures that data won't get out-of-synch,
however, it also makes the data unavailable should a user of the transactional system need to view
or update it.
A data set is a subset of a database or a data warehouse. It is usually denormalized so that only
one table is used. The creation of a data set may contain several steps, including appending or
combining tables from source database tables, or simplifying some data expressions. One example
of this may be changing a date/time format from '10-DEC-2002 12:21:56' to '12/10/02'. If this