A trigger fires once per SQL statement regardless of how many records are affected by the statement.
You may want or required to take some action in a trigger when 0 records are affected.

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>I was looking at using the 'INSTEAD OF' trigger but this in an AFTER is simpler. The only thing that threw me for a while was that the trigger fired when there was nothing to insert (ie INSERT INTO ...... (SELECT ......) caused the trigger to fire even if the select returned no records - that didn't seem very logical to me?
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