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   RE: [xml-dev] Are people really using Identity constraints specified in

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Michael Kay wrote:

> I tend to be a little wary of constraints myself. 
> Many of those you see in student textbooks are 
> misguided. If I see a schema (XML or RDB) with the
> constraint that employees must be over 16, I ask 
> myself what the IT department would do if the 
> business decided to hire someone under 16. If
> there's a rule that an employee's manager must 
> themselves be an employee, I ask what would 
> happen when someone is told that they now report 
> to a contractor. 

This is excellent:

> It's not the job of computers to limit what people 
> are allowed to do (or the job of the IT department 
> to regulate the business). 

The following innocuous sentence has profound implications  
on the role of schemas:

> A guideline I use is that constraints should be there 
> only to protect the IT system itself from data that 
> it cannot handle.

Would you elaborate upon this sentence Michael?  I believe
that you are saying that the role of a schema is to define
things such as:
- ensure that a "date" is indeed a valid date
- ensure that an "age" is indeed a valid age

The role of a schema is not, for example, to specify:
- the "age" must be at least 16.

So, your guideline says: use schemas to specify datatypes
for objects, not their range of values.  Is that a fair
summary of your guideline?  /Roger


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