>you can either do
>select a,b,c from mytable group by a,b,c;
>or
>select a,b from mytable group by a,b;
>
>but not
>select a,b,c from mytable group by a,b;
>
>Sorry if that's not the answer you are looking for...

But you can do this:

select a,b, aggregate_function(c) from mytable group by a,b;

Where aggregate_function is something like sum(), min(), max(), count(), etc.