sql-porting-problem oracle to postgresql with UPDATE/IS NOT NULL

hello list,
i have some problems with an sql-statement which runs on oracle but not on
postgresql (i want update only if result of SELECT is not empty, the
SELECT-queries are identical):

UPDATE table1 t1
SET (t1.id) =
(SELECT h.id FROM table2 t2,table3 t3, table1 t1
WHERE t3.field = t2.field
AND t2.id = t1.id
AND t1.id <> t3.id)
WHERE
(SELECT h.id FROM table2 t2,table3 t3, table1 t1
WHERE t3.field = t2.field
AND t2.id = t1.id
AND t1.id <> t3.id) IS NOT NULL;

thanks, hans