>>>>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

Agree - the problem turned out to be too complex :(