date null

Hi, how can control in a "select" if a date is not null?
I have this table:
Table = prestito
+----------------------------------+----------------------------------+-------+
| Field | Type |
Length|
+----------------------------------+----------------------------------+-------+
| id_libro | varchar() not null |
10 |
| id_utente | int4 not null |
4 |
| data_prestito | date |
4 |
| data_restituzione | date |
4 |
| n_gg_prestito | int4 not null |
4 |
| notifica1 | date |
4 |
| notifica2 | date |
4 |
| notifica3 | date |
4 |
+----------------------------------+----------------------------------+-------+

and I want to select only tuples in which notifica1 or notifica2 or
notifica3 are not null.

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