Trouble in SELECT query

I have a PostgreSQL 7.1. When I start query like "SELECT * FROM table1 WHER=
E field1 =3D NULL;" it working OK, but query like "SELECT * FROM table1 WHE=
RE field1 <> NULL" returns the void result. Field "field1" has type DATE. "=
... field1 IS NULL", "... field1 IS NOT NULL", "... field1 <> 0" also worki=
ng correctly. "... field1 =3D 0" also not working.

        With best regards, Serg Matskov.