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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Are people really using Identity constraints specified in
 >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. Michael Kay 
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