In the July 2004 issue of DM Review, I discussed the concept of dimensional placeholders. I mentioned that you should not allow a null in any part of the fact table key. This month, I would like to provide an example demonstrating how this can cause problems that are rather difficult to debug. To illustrate the problem, let's look at two dimensions on a fact table. In reality, you will have more than two dimensions, and, likewise, the problem can become much more complex. Consider the simple sales example in Figure 1.
Figure 1: Simple Sales Example
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