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>I am a research student. I am working on the project which will convert
>identity constraints specified in XML schema to relational keys while
>mapping XML to relations.
The biggest problem here is that the scope of a database and a document are
different. A document described by an XML schema usually describes one
business object, whereas a relational database describes many. Most of the
constraints in a relational database are cross-object constraints, whereas
XML schema can only describe intra-object constraints.
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. 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). A
guideline I use is that constraints should be there only to protect the IT
system itself from data that it cannot handle.