>>>>>>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 :(
>
>It is not complex as long as one tests a solution prior to posting it

It is complex. Re-read my other post. I see a simple way of solving it in SQL Server, but I don't see a simple way of solving it in one select in VFP.