A fact table in data warehousing contains measures, metrics of a business and dimensional surrogate keys surrounded by dimension tables in a Star/Snow-flake schemas. There are three types of measures
- Additive – Measures that can be added across all dimensions.
- Non Additive – Measures that cannot be added across any dimension.
- Semi Additive – Measures that can be added across some dimensions and not across others.
Types of fact tables
There are basically three fundamental measurement events,
- A transactional table is the most basic and fundamental. The grain associated with a transactional fact table is usually specified as “one row per line in a transaction”, e.g., every line on a receipt. Typically a transactional fact table holds data of the most detailed level, causing it to have a great number of dimensions associated with it.
- Periodic snapshots
- The periodic snapshot, as the name implies, takes a “picture of the moment”, where the moment could be any defined period of time, e.g. a performance summary of a salesman over the previous month. A periodic snapshot table is dependent on the transactional table, as it needs the detailed data held in the transactional fact table in order to deliver the chosen performance output.
- Accumulating snapshots
- This type of fact table is used to show the activity of a process that has a well-defined beginning and end, e.g., the processing of an order. An order moves through specific steps until it is fully processed. As steps towards fulfilling the order are completed, the associated row in the fact table is updated. An accumulating snapshot table often has multiple date columns, each representing a milestone in the process. Therefore, it’s important to have an entry in the associated date dimension that represents an unknown date, as many of the milestone dates are unknown at the time of the creation of the row.
Factless Fact table: A fact table contains no measures is called a fact less fact table, i.e., Fact table contains only the forieng keys of dimensions/fact tables.