RE: Are people really using Identity constraints specified in
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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