创建一格database link 名称为 link_scott，连接scott用户，scott用户密码为tiger，
连接的ip是192.168.1.2

-- Create database link 
 create database link  link_scott
   connect to scott
   identified by tiger
   using '(DESCRIPTION =(ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.1.2)(PORT = 1521))
 (CONNECT_DATA =(SERVER = DEDICATED)(SERVICE_NAME = orcl))
  )';

 
 创建之后，就可以跨库访问scott用户下的表了
 比如scott用户下有 dmp这个表，查询dmp表下所有数据

select * from dmp@link_scott