Questo tipo di codice forse dovrebbe funzionare per te
SELECT
*,
CASE
WHEN (pvc IS NULL OR pvc = '') AND (datepose < 1980) THEN '01'
WHEN (pvc IS NULL OR pvc = '') AND (datepose >= 1980) THEN '02'
WHEN (pvc IS NULL OR pvc = '') AND (datepose IS NULL OR datepose = 0) THEN '03'
ELSE '00'
END AS modifiedpvc
FROM my_table;
gid | datepose | pvc | modifiedpvc
-----+----------+-----+-------------
1 | 1961 | 01 | 00
2 | 1949 | | 01
3 | 1990 | 02 | 00
1 | 1981 | | 02
1 | | 03 | 00
1 | | | 03
(6 rows)