请教一个sql语句的含义，谢谢各位！

Question

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？

Answer 1

最右边的where是update的条件，中间的where是给id set值的条件