>>>>>>>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.

see message#1416401 for a simple way in vfp