savepoint a;
delete from student ;
rollback to a;
Oracle中修改日期格式
alter session set nls_date_format='yyyy-mm-dd'
Oracle中查询字段为空或不为空的记录
select * from student where birthday is null
select * from student where birthday is not null
删除配置文件
drop profile a [cascade]
alter user xh account unlock：为用户解锁
限制用户登录失败的次数
1.建立配置文件，用户登录失败三次就锁二天，黑体为关键字：
create profile a limit failed_login_attempts 3 password_lock_time 2
2.把这个配置文件运用到用户xh
alter user xh profile a