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

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  • To: xml-dev@lists.xml.org
  • Subject: RE: [xml-dev] Are people really using Identity constraints specified in XML schema?
  • From: "Prakash, Prakash Yamuna, Yamuna" <techpy@yahoo.com>
  • Date: Sun, 22 Aug 2004 23:28:13 -0700 (PDT)

>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.

True policies can can change - but does it imply constraints are not valid? The constraints are driven by the organizational policies. If the organizational policies require that employees must be over 16, then they need to be enforced. If the organizational policies change of course the constraint needs to be dropped or modified. All it implies to me is that constraints have a validity period i.e. these are not invariant constraints. While perhaps datatyping of fields are more invariant in nature...


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