Fact Table | Star Schema:
The centralized table in a star schema is called as FACT table. A fact table typically has two types of columns: those that contain facts and those that are foreign keys to dimension tables. The primary key of a fact table is usually a composite key that is made up of all of its foreign keys.
In the example, “Sales Dollar” is a fact (measure) and it can be added across several dimensions. Fact tables store different types of measures like additive, non additive and semi additive measures.
- Additive – Measures that can be added across all dimensions.
- Non Additive – Measures that cannot be added across all dimensions.
- Semi Additive – Measures that can be added across few dimensions and not with others.
In the real world, it is possible to have a fact table that contains no measures or facts. These tables are called as Factless Fact tables.
Steps in designing Fact Table:
- Identify a business process for analysis(like sales).
- Identify measures or facts (sales dollar).
- Identify dimensions for facts(product dimension, location dimension, time dimension, organization dimension).
- List the columns that describe each dimension.(region name, branch name, region name).
- Determine the lowest level of summary in a fact table(sales dollar).
Example of a Fact Table with an Additive Measure in Star Schema: