>>Unfortunately, this would not give us only error cases - I just thought of the same SQL. You don't need to join here. This would give all C3 cases even when a/b combination is the same - I don't see a simple way to separate them - need to think.
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>Yes, and start reading the question first
BTW, I re-read the question and your select statement was not solving it.
If it's not broken, fix it until it is.
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