Hola, mira si te sirve esto:
Código:
select case desglodoc when 1 coalesce(sum(imporfac-ivadoc),0) else coalesce(sum(imporfac),0) end as total,f_month(FECHAFAC) from facturas
where tipomov='FM'
and fechafac >='01.01.2003'
and fechafac <='28.02.2003'
and (tasaiva=1 or tasaiva=3 or ivadoc>0)
group by f_month(FECHAFAC)
order by f_month(FECHAFAC)
Saludos,
Jorge.