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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