用户是否存在：  select   *   from   all_users   where   username   =  ‘...’
                  select   *   from   dba_users   where   username   =  ‘...’


  主机名:   select   host_name   from   v$instance
   
  客户机名: select   SYS_CONTEXT('USERENV','HOST')from   dual  
   
  主机IP: select   utl_inaddr.get_host_address   from   dual  
   
  客户机IP: select   SYS_CONTEXT('USERENV','IP_ADDRESS')   from   dual