Fact and Dimension Examples

A fact table works with dimension tables. A fact table holds the data to be analyzed, and a dimension table stores data about the ways in which the data in the fact table can be analyzed. Thus, the fact table consists of two types of columns. The foreign keys column allows joins with dimension tables, and the measures columns contain the data that is being analyzed.

Suppose that a company sells products to customers. Every sale is a fact that happens, and the fact table is used to record these facts. For example:

 Fact Table Example:

Time ID	Product ID	Customer ID	Unit Sold
4	17	2	1
8	21	3	2
8	4	1	1

 Now we can add a dimension table about customers:

 Dimension Table Examples:

Customer Dimension

Customer ID	Name	Gender	Income	Education	Region
1	Brian Edge	M	2	3	4
2	Fred Smith	M	3	5	1
3	Sally Jones	F	1	7	3

 Date Dimension

Time ID	DateKey	Date_UK	Date_USA	DayofMonth	DayName
1	20130101	01/01/2013	01/01/2013	1	Tuesday
2	20130102	02/01/2013	02/01/2013	2	Wednesday
3	20130103	03/01/2013	03/01/2013	3	Thursday

  Product Dimension

Product ID	Product Name	Product Business Key	Batch ID	Category	Group
1	Aero Milk	AC-3AA	1	Dairy	Dairy
2	Bikky Rice	BK-B34	2	Food	Cereals
3	Bikky Bics	BZ-CG5	2	Biscuits	Cookies

In this example, the customer ID column in the fact table is the foreign key that joins with the dimension table. By following the links, you can see that row 2 of the fact table records the fact that customer 3, Sally Jones, bought two items on day 8. The company would also have a product table and a time table to determine what Sally bought and exactly when.

When building fact tables, there are physical and data limits. The ultimate size of the object as well as access paths should be considered. Adding indexes can help with both. However, from a logical design perspective, there should be no restrictions. Tables should be built based on current and future requirements, ensuring that there is as much flexibility as possible built into the design to allow for future enhancements without having to rebuild the data.