Question: What are junk dimensions and degenerate dimensions and what is the importance of junk and degenerate dimensions in a data warehouse?
Chuck Kelley's Answer: Junk dimensions are dimensions that contain miscellaneous data (like flags and indicators) that do not fit in the base dimension table. Here is a link to Ralph Kimball and Margy Ross on junk dimensions. http://www.kimballgroup.com/html/designtipsPDF/DesignTips2003/KimballDT48DeClutter.pdf
A degenerate dimension is data that is dimensional in nature but stored in a fact table. For example, if you have a dimension that only has Order Number and Order Line Number, you would have a 1:1 relationship with the Fact table. Do you want to have two tables with a billion rows or one table with a billion rows. Therefore, this would be a degenerate dimension and Order Number and Order Line Number would be stored in the Fact table. Here is a pointer to this question from a previous ATE column. http://www.dmreview.com/article_sub.cfm?articleId=7844