>>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

I believe we wanted to get all records where we have discrepancy. BTW, I figured this out while driving.

select T.* from myTable T INNER JOIN (select a,b,c from myTable group by c, a, b) X on T.c= X.c and (T.a<>X.a OR T.b <>X.b)