查看当前登陆的用户名：
SQL> select user from dual;

USER
------------------------------
SYS

SQL> show user;
USER is "SYS"
SQL> select username from user_users;

USERNAME
------------------------------
SYS

SQL>

查看当前用户拥有的角色和权限：
select * from user_role_privs;
select * from session_privs;