Step-4: Explanation of Slowly Changing Dimensions…

• Compared to fact tables, contents of dimension tables are relatively stable.
  • New sales transactions occur constantly.
  • New products are introduced rarely.
  • New stores are opened very rarely.

• The assumption does not hold in some cases
  • Certain dimensions evolve with time
  • e.g. description and formulation of products change with time
  • Customers get married and divorced, have children, change addresses etc.
  • Land changes ownership etc.
  • Changing names of sales regions.

For example, a minor ingredient change or a minor packaging change may be so small that production does not assign the product a new SKU number (which the data warehouse has been using as the primary key in the product dimension), but nevertheless gives the data warehouse team a revised description of t he product. The data warehouse team faces a dilemma when this happens. If they want the data warehouse to track both the old and new descriptions of the product, what do they use for the key? And where do they put the two values of the changed ingredient attribute?

Other common slowly changing dimensions are the district and region names for a sales force. Every company that has a sales force reassigns these names every year or two. This is such a common problem that this example is something of a joke in data ware housing classes. When the teacher asks, "How many of your companies have changed the organization of your sales force recently?" everyone raises their hands.

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