Databases Reference
In-Depth Information
Chapter 13
A d d i t i o n a l N o r m a l F o r m s
Where's it all going to end?
─Tom Stoppard: Rosencrantz and Guildenstern Are Dead
Now, this is not the end. It is not even the beginning of the end.
But it is, perhaps, the end of the beginning.
─Winston Churchill: The End of the Beginning
To paraphrase something I said in Chapter 9, I've assumed so far in this topic that the only dependencies we care
about 1 are ones that have to do with projection as the decomposition operator and join as the corresponding
recomposition operator. I also said that, given that assumption, it followed that 5NF was the final normal form.
However, I did also say, in a footnote, that there was something called “sixth” normal form or 6NF. In fact, it turns
out that we can define, not just 6NF as such, but several other normal forms also, all without departing from those
same assumptions regarding available decomposition and recomposition operators. Fig. 13.1 (an extended version
of Fig. 3.3 from Chapter 3) shows how some of those additional normal forms─viz., RFNF, SKNF, and 6NF, shown
in boldface italics in the figure─fit into the overall scheme of things, as it were. In this chapter, I'll be describing
those three normal forms as well as (briefly) a few more, for completeness.
1NF
2NF
3NF
BCNF
4NF
RFNF
SKNF
5NF
6NF
Fig. 13.1: The normal form hierarchy (II)
EQUALITY DEPENDENCIES
Before describing the various additional normal forms as such, I need to spend a little time on another preliminary
matter. Recall from Chapter 3 the example in which relvar S was replaced by its projections SNC and CT on
1 Apart from equality and inclusion dependencies, that is.
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