Re: two nasty schemata, union types and surrogate keys
Date: Mon, 5 Oct 2009 17:45:00 +0000 (UTC)
>> Normalization has very little to do with the semantics of the data. If
>> you'll look carefully at all the examples of converting from one normal form
>> to the next higher form, you'll see that the schema before conversion and
>> the one after conversion are equivalent in their capacity to express facts.
>> That's important.
>And it's wrong! Normalization has everything to do with the semantics
>of the data. [...]
>In the 3NF scheme, it is possible for the fact that a supplier has a
>particular phone number to be expressed even if the supplier doesn't
>at present supply any parts. Every fact that can be expressed in the
>2NF schema can also be expressed in the 3NF scheme, but not every fact
>in the 3NF scheme can be expressed in the 2NF schema.
This is because you're dropping an inclusion dependency when going to the 3NF version.
More generally, the reason you're disagreeing is that people seem to disagree what exactly it means to normalize or denormalize: they agree on what happens to the table structure, but not on what happens to constraints. E.g. the Wikipedia articles on this subject are sorely lacking on this matter.
>> Contrary to popular belief, adding a surrogate key to each table does not
>> change the level of normalization of a schema. Candidate keys remain
>> candidate keys after a surrogate has been added and declared the primary
>> key. I'm not sure, but I think you're asserting the opposite above.
>> How do you know that there are two sets of cars owned, with each set owned
>> by a different person, instead of one set consisting of two cars and with
>> one owner? Is that fact expressed in the database? How?
>If you add a surrogate, then the fact that the surrogates are
>different is sufficient to determine that there is more than one
Tuples don't represent objects, but statements of fact. Different statements about persons may need to be recorded in situations where it is not known whether they pertain to the same person. E.g. in some kind of observation database (say, incident reporting on traffic) this may be necessary. Doing so doesn't violate the closed world assumption.
-- ReinierReceived on Mon Oct 05 2009 - 19:45:00 CEST