L' Eomot Inversé (4/25/2013)

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Good question and explanation.

Triggers that work only on single rows are a common problem, so anything that points out the pitfalls and warns people not to write them is a good thing.

But I have a minor quibble: if this is a business rule, it should be enforced by the schema if possible, not by code; and this is a classic example of a rule that can be enforced by a check constraint.