Draft Design Exercise Discussion
Now that you’ve reviewed the common dimensional modeling mistakes frequently encountered during design reviews, refer to the draft design in Figure 11-2. Several opportunities for improvement should immediately jump out at you.
The first thing to focus on is the grain of the fact table. The team stated the grain is one row for each bill each month. However, based on your understanding from the source system documentation and data profiling effort, the lowest level of billing data would be one row per service line on a bill. When you point this out, the team initially directs you to the bill dimension, which includes the service line number. However, when reminded that each service line has its own set of billing metrics, the team agrees the more appropriate grain declaration would be one row per service line per bill. The service line key is moved into the fact table as a foreign key to the service line dimension.
While discussing the granularity...