$sqlplus  / as sysdba

1. 查询oracle中所有用户信息

select * from dba_users;
select * from all_users; 
select distinct owner from all_objects;

2. 只查询用户和密码

select username,password from dba_users;

3. 查询当前用户信息

select * from dba_ustats;

4. 查询用户可以访问的视图文本

select * from dba_varrays;

5. 查询数据库中所有视图的文本

select * from dba_views;