I've been reviewing my posts on the forums and came across this one, so I thought I could share my solution with the rest of you... someone might hopefully find it helpful one day:well, it never worked with ST_intersects, so I've switched to St_Contains and that one seemed to work. Then I've used the EXISTS keyword, which has later proved to be a crucial part of the statement.
UPDATE stat.AP x SET x.is_alloc = 1, x.SO_ID = 555 WHERE ((x.ZSJ_ID = 353) AND EXISTS(SELECT y.objectid FROM stat.SO y WHERE sde.ST_Contains(y.shape, x.shape) = 1))
this might be a slightly different to what I initially wrote in my first post, but the idea is there