Prova ad aggiungere le parentesi
codice:
SELECT *,date_format(datafattura, '%d-%m-%Y') as data_it,
date_format(datascadenza, '%d-%m-%Y') as data_it2, DATEDIFF(CURDATE(), datascadenza) AS data_diff
FROM fatture, clienti
WHERE fatture.idcliente = clienti.idcliente and (tipopagamento = '30' or tipopagamento = '60' or tipopagamento = '90') order by data_diff DESC
oppure usa in
codice:
SELECT *,date_format(datafattura, '%d-%m-%Y') as data_it,
date_format(datascadenza, '%d-%m-%Y') as data_it2,
DATEDIFF(CURDATE(), datascadenza) AS data_diff
FROM fatture, clienti
WHERE fatture.idcliente = clienti.idcliente and tipopagamento in (30,60,90)
order by data_diff DESC