codice:
select count(*) as quanti,
case 
when eta_clienti <= 24 then "minore 24"
when eta_clienti between 25 and 34 then "25-34"
when eta_clienti between 35 and 44 then "35-44"
when eta_clienti between 45 and 54 then "45-54"
when eta_clienti > 54 then "> 54"
end as fascia
from clienti as c
inner join proced as p
on c.id_clienti = p.ref_proced
where p.data_proced != '0000-00-00'
group by fascia