update xj_ftthSN_MSG a set (a.city_id) = (select b.city_id from tab_deviceresource b where a.oltip = b.loopback_ip
and rownum < 2)
where exists (select 1 from tab_deviceresource c where a.oltip = c.loopback_ip)
oracle中可以执行
请分析一下，这条语句的执行策略和
为什么这样写能执行成功？
