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

["Gau, Hans-Jürgen" <Hans-Juergen(dot)Gau(at)LGN(dot)Niedersachsen(dot)de>]

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