Database Modeling and Design

156 CHAPTER 8 Business Intelligence
operational system environment lead to distinct design approaches.
Dimensional modeling is the guiding principle in data warehouse
design.
Snowflake Schema
The data warehouse literature often refers to a variation of the star
schema known as the snowflake schema. Normalizing the dimension
tables in a star schema leads to a snowflake schema. Figure 8.4 shows the
snowflake schema analogous to the star schema of Figure 8.3. Notice
that each hierarchical level becomes its own table. The snowflake
schema is generally losing favor. Kimball and Ross strongly prefer the
star schema, due to its speed and simplicity. Not only does the star
schema yield quicker query response, it is also easier for the user to
understand when building queries. We include the snowflake schema
here for completeness.
Dimensional Design Process
We adhere to the four-step dimensional design process promoted by Kimball
and Ross. Figure 8.5 outlines the activities in the four-step process.
Dimensional Modeling Example
Congratulations, you are now the owner of the ACME Data Mart Company!
Your company builds data warehouses. You consult with other
companies, design and deploy data warehouses to meet their needs, and
support them in their efforts.
Your first customer is XYZ Widget, Inc. XYZ Widget is a manufacturing
company with information systems in place. These are operational
systems that track the current and recent state of the various business
processes. Older records that are no longer needed for operating the
plant are purged. This keeps the operational systems running efficiently.
XYZ Widget is now ten years old, and growing fast. The management
realizes that information is valuable. The CIO has been saving data
before they are purged from the operational system. There are tens of
millions of historical records, but there is no easy way to access the data
in a meaningful way. ACME Data Mart has been called in to design and
build a DSS to access the historical data.
Discussions with XYZ Widget commence. There are many questions
they want to have answered by analyzing the historical data. You begin
by making a list of what XYZ wants to know.