Dimensional Modeling (DM) tutorial with OLAP and data warehouse design concepts

This dimensional modeling tutorial is intended for people with basic SQL and relational database design skills.

Data Management - Dimensional Modeling Definition

Dimensional modeling is the design concept used by many data warehouse designers to build their data warehouse. Dimensional model is the underlying data model used by many of the commercial OLAP products available today in the market. In this model, all data is contained in two types of tables called Fact Table and Dimension Table.

Dimensional Modeling - Fact Table

In a Dimensional Model, Fact table contains the measurements or metrics or facts of business processes. If your business process is Sales, then a  measurement of this business  process such as "monthly sales number" is captured in the fact table. In addition to the measurements, the only other things a fact table contains are foreign keys for the dimension tables.

Dimensional Modeling - Dimension Table

In a Dimensional Model, context of the measurements are represented in dimension tables. You can also think of the context of a measurement as the characteristics such as who, what, where, when, how  of a measurement (subject ). In your business process Sales, the characteristics of the 'monthly sales number' measurement can be a Location (Where), Time (When), Product Sold (What). The Dimension Attributes are the various columns in a dimension table. In the Location dimension, the attributes can be Location Code, State, Country, Zip code. Generally the Dimension Attributes are used in report labels, and query constraints such as where Country='USA'. The dimension attributes also contain one or more hierarchical relationships.Before designing your data warehouse, you need to decide what this data warehouse contains. Say if you want to build a data warehouse containing monthly sales numbers across multiple store locations, across time and across products then your dimensions are:

1. Location
2. Time
3. Product

Each dimension table contains data for one dimension. In the above example you get all your store location information and put that into one single table called Location. Your store location data may be spanned across multiple tables in your OLTP system (unlike OLAP), but you need to de-normalize all that data into one single table.