create database link DATABASE_LINK_TEST--数据库别名
  connect to DB_TEST identified by DB_TEST--分别对应用户名和密码
  using '127.0.0.1:1521/ORCL';--域名:端口号/数据库实例

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_TEST --自定义要连接的数据库名称
  connect to username identified by password  --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 = 127.0.0.1)(PORT = 1521))
    (CONNECT_DATA =
      (SERVICE_NAME = orcl)
　　第二步：调用方式
　　B库表名@连接的B数据库的别名
select * from BASE_TEST@DATABASE_LINK_TEST