create database link 随便起个名字
  connect to 用户名 identified by 密码
  using ‘(DESCRIPTION =                       
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 域名)(PORT = 端口号))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = 数据库实例名称)
    )
  )‘;

-- Create database link 
create database link DATABASE_LINK_DW_XNH --自定义要连接的数据库名称
  connect to DW_XNH identified by DW_XNH  --用户名和密码
  using ‘(DESCRIPTION =                      --数据库连接：域名，端口号，数据库实例    
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.2)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = orcl)
    )
  )‘;

select * from BASE_TEST@DATABASE_LINK_DW_XNH