查看oracle数据库的连接数以及用户

1、查询oracle的连接数 
select count(*) from v$session; 
2、查询oracle的并发连接数 
select count(*) from v$session where status='ACTIVE'; 
3、查看不同用户的连接数 
select username,count(username) from v$session where username is not null group by username; 
4、查看所有用户： 
select * from all_users; 
5、查看用户或角色系统权限(直接赋值给用户或角色的系统权限)： 
select * from dba_sys_privs; 
select * from user_sys_privs; 
6、查看角色(只能查看登陆用户拥有的角色)所包含的权限 
select * from role_sys_privs; 
7、查看用户对象权限： 
select * from dba_tab_privs; 
select * from all_tab_privs; 
select * from user_tab_privs; 
8、查看所有角色： 
select * from dba_roles; 
9、查看用户或角色所拥有的角色： 
select * from dba_role_privs; 
select * from user_role_privs; 
10、查看哪些用户有sysdba或sysoper系统权限(查询时需要相应权限) 
select * from V$PWFILE_USERS;