codice:
set @num=0;
select ....,sum(importo) as totale, @num := @num + 1 as riga_numero
from .....
group by .....
order by totale desc
Per una riga specifica aggiungi la clausola having

codice:
set @num=0;
select ....,sum(importo) as totale, @num := @num + 1 as riga_numero
from .....
group by .....
having riga_numero = 20
order by totale desc