>>>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.
>>
>>Yes, and start reading the question first
>
>BTW, I re-read the question and your select statement was not solving it.

I have no time for this