[postgis-users] subqueries in update?

[Marc Weber]

Beeing a total noob I wonder wether I can use suqueries such like 
update haueser set inside=contains(geomunion( (SELECT the_geom from shape) ),the_geom);

to set a property of each row indicating that the polygon is inside the union of all polygons of table shape.

Reading posgres documentation I only could find subqieries in the Where or
SELECT ... part of a query.

If not how would you accomplish this task?

Marc