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From Aristedes Maniatis <...@maniatis.org>
Subject Natural primary keys, Was: Cayenne with JSF 2
Date Wed, 04 Apr 2012 09:32:03 GMT
On 4/04/12 7:06 PM, Durchholz, Joachim wrote:
>> However any time you have a primary key with meaning is probably
>> a mistake in database design. You are best trying to avoid this
>> at all costs unless you are constrained by a legacy system in
>> some way.
>
> This is common advice, but the topic is controversial.
> Even in non-legacy databases, having natural PKs can have advantages:

[snip]

I take your points, but almost anything that looks like a natural key... rarely is. Back when
I developed systems for other people, I could not count the number of times a customer guaranteed
that a certain value would never ever change. Ever. And of course it did.

People who aren't programmers have a more relativistic concept of "never". To them, getting
hit by lighting is "something that just never happens". If it happens once every 5 years,
that might also qualify as "never". To them, your question simply means "so rare as to be
not important to their daily lives". But of course there is a situation when that EAN isn't
unique. When an ISBN is accidentally used twice for the casebound and software editions (they
should be different). For a programmer unique and immutable have a special absolute meaning.

Yes a government tax number or social security number is unique and immutable. And that would
be true only if your database table was a table representing social security numbers. Instead
it probably represents 'people' and then the correlation becomes far less certain. Do government
departments ever make mistakes? Do they reassign numbers to people in a witness protection
program?


Maybe there are exceptions. I can't think of any. At any rate, once you give yourself over
to an ORM (Cayenne or anything else), the agreement you make with the software is "Give me
objects and don't bother me with how they are stored." SQL with a primary key or 12 chimpanzees
with punch cards, it doesn't matter. Sure you can open the hood and tinker with the engine.
But if you just want to just drive the car, primary keys are not something that should be
exposed in your Java code.


Ari



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Aristedes Maniatis
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