database link是定义一个数据库到另一个数据库的路径的对象，database link允许你查询远程表及执行远程程序。只有建立该database link的schema的session能使用这个database link来访问远程的数据库。-Database link is a definition of a database to another database path object, database link allows you to query remote table and implementation of remote program. Only by establishing the database link of schema session can use the database link to access the remote database.