创建database link
基本语法
CREATE [ SHARED ][ PUBLIC ] database link link_name
      [ CONNECT TO user IDENTIFIED BY password ]
      [ AUTHENTICATED BY user IDENTIFIED BY password ]
      [ USING ‘ connect_string ‘ ]
 
 例子：
create database link DL
  connect to username identified by password
  using ‘191.168.1.105:1521/ORCL‘;

使用方法：
select * from road@DL t