a jednak się da w jednym zapytaniu :)

UPDATE nodes n1,(
SELECT n.id
  FROM nodes n
   LEFT JOIN nodeassignments ON n.id = nodeassignments.nodeid
   LEFT JOIN assignments ON nodeassignments.assignmentid=assignments.id
   WHERE (assignments.dateto > unix_timestamp() or assignments.dateto='0' )
   AND assignments.datefrom < unix_timestamp()
   AND assignments.suspended = 0
   AND access = '0'
   AND (SELECT SUM(value) FROM cash WHERE customerid = n.ownerid) >= 0
) AS n2
SET n1.access=1, n1.warning=0
WHERE n1.id = n2.id

i to mi wyłapuje jak narazie dobrze :)