4.6. Projections and Joins
The previous section showed that a sufficient condition for splitting an unnested fact type is that it has a uniqueness constraint spanning fewer than n – 1 roles. Since this is not a necessary condition for splitting, in some cases further analysis is required to make a definite decision. At the heart of such an analysis is the question: can the fact type be rephrased as a conjunction of smaller fact types?
With a bit of experience behind us, we can usually answer this question fairly quickly. This section discusses a formal procedure for addressing this question in a systematic way. The procedure makes use of two operations known as “projection” and “join”, which are important in relational database work. The join ...
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