注意点:1.被update主表一定要加上过滤条件.2.查询出来更新结果集,同时也要作为被更新主表的条件,作为同步大家都是更新这部分数据.update student stu set (stu.name,stu.sex) = (select bak.name,bak.sex from student_bak bak where bak.id = stu.id and stu.age = 20 ) where exists (select 1 from student_bak bak where b