James Goodwin (7/28/2011)

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Tom,
Thanks for the series, it has been very interesting thus far.

However, I think more distinction needs to be made between 2NF and 3NF. The example seems to be about 2NF. The Candidate key for the original table is City. The Phone number is not dependent upon City at all, it is dependent upon Group. I think that you are trying to show a transitive relationship from the Key to the phone number. But that relationship doesn't seem to exist in the example.

In the wikipedia example of 3NF http://en.wikipedia.org/wiki/Third_normal_form this transitivity is expressed by having two overlapping candidate keys, one of which determines the non-prime attribute, but the other doesn't. This seems to imply that the difference between 2NF and 3NF is that in 2NF non-prime attributes need to depend on A candidate key, while in 3NF the non-prime attributes need to depend on ALL candidate keys.

So, can you please expand on this and clear up my confusion?

Thanks,
JimFive

roger.plowman (7/28/2011)

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Due to the extreme flexibility requirements of our reporting system users need at least a nodding acquaintence with the database schema--primarily how specific data can be found in specific tables, and how the tables are related to each other.

Rather than try and explain anything about normal forms we use the SPOT and Path metaphor. SPOT = Single Point Of Truth (ie the data is only stored in one single place *once*) and there are always paths between tables, so for example a Zone (set of contiguous zip codes) is always connected to only one Office which in turn is in one Region.

Of course the data has to be 3NF to support this, but I always thought SPOT and Path was a much more natural way to visualize 3NF anyway.

Michael.Beeby (7/28/2011)

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NB: 40 Tree Rd became 30 Tree Rd at some point.