uno modo protrebbe essere cosi:

codice:
select
entrate.entrata_id       as id,
entrate.entrata_valore   as v_entrata,
cast(0 as numeric(15,2)) as v_uscita,
entrate.entrata_data     as data,
entrate.utente_id
from entrate

union all

select
uscite.uscita_id         as id,
cast(0 as numeric(15,2)) as v_entrata,
uscite.uscita_valore     as v_uscita,
uscite.uscita_data       as data,
uscite.utente_id
from uscite