1.查看所有用户：
select * from dba_users;
select * from all_users;
select * from user_users;

2.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;

3.查看角色(只能查看登陆用户拥有的角色)所包含的权限
sql>select * from role_sys_privs;

4.查看用户对象权限：
select * from dba_tab_privs;
select * from all_tab_privs;
select * from user_tab_privs;

5.查看所有角色：
select * from dba_roles;

6.查看用户或角色所拥有的角色：
select * from dba_role_privs;
select * from user_role_privs;

7.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

8.查看一个用户所有的权限及角色
select privilege from dba_sys_privs where grantee=’username’
union
select privilege from dba_sys_privs where grantee in
(select granted_role from dba_role_privs where grantee=’username’ );