Facts vs. Dimensions in Data Warehousing

Based on “The Microsoft Data Warehouse Toolkit” by Joy Mundy and Warren Thornthwaite,

“It may help to think of dimensions as things or objects. A thing such as a product can exist without ever being involved in a business event.”

As the book describes, a dimension is your noun. It is something than can exist independent of a business event, such as a sale. Examples of things that can exist are Products, employees, equipment, etc. A dimension either does something, or has something done to it.

Examples: Customers buy and employees sell. In this case, Customers and Employees are dimensions.

Facts are the verbs. An entry in a fact table makes a discrete event that happens to something from the dimension table.

  • A product sale would be recorded in a fact table.
  • Product, Employee, and Customer are all dimensions that describe the event, the sale.

In addition fact tables also typically have some kind of quantitative data. The quantity sold, the price per item, total price, and so on.