Data Warehouse: Facts and Measures

What is a Fact Table?

A fact table is a table that joins dimension tables with measures.  For example, Lets say you wanted to know the time worked by employees, by location, by project and by task.  If you had a dimension for employees, location, project and task you would create a composite primary key using these foreign keys and add an additional column for the time worked measure. (more on measures in a little bit)

Keep in mind that fact tables are huge unlike dimension tables.  Fact tables are usually built to contain a hundred thousand records on the low side up to billions of records on the high side.  Therefore, these tables must be normalized to be efficient.

A fact table is normalized when it consists of the following two things:

1. 1. A composite primary key using the foreign keys to the dimension tables.
   2. Measure(s) used for numerical analysis.

What is a measure?

A measure is normally an additive numerical value that represents a business metric.  You are also not limited to a single measure.  You can have multiple measures within a fact table.  For example, if your fact table is used to track purchases internationally you might have measures for each type of currency.  If you are building a fact table for the retail industry you might also have the following measures cost, list price, average sale price.

Source: http://johnsterrett.com/2009/11/05/data-warehouse-facts-and-measures/