SQL

[marketto]

prova così:

SELECT t1.id_Clienti, t1.data_ordine FROM ordini AS t1
INNER JOIN ordini AS t2 ON t1.id_Clienti = t2.id_Clienti
WHERE t2.data_ordine IS NULL

oppure:
[code]

SELECT t1.id_Clienti, t1.data_ordine FROM ordini AS t1
INNER JOIN ordini AS t2 ON t1.id_Clienti = t2.id_Clienti
WHERE t2.data_ordine = ''