1.
SYS用户具有DBA权限，并且拥有SYS模式，只能通过SYSDBA登陆数据库。是Oracle数据库中权限最高的帐号
SYSTEM具有DBA权限。但没有SYSDBA权限。平常一般用该帐号管理数据库就可以了。
2.
（1）sys -- sysdba
system --sysoper
（2）startup/shutdown/dba管理两个都能做

（3）sys拥有数据字典(dictionay),或者说dictionay属于sys schema

（4）sysdba比sysoper多了一个create database的权限
3.
SYSDBA
Perform STARTUP and SHUTDOWN operations
ALTER DATABASE: open, mount, back up, or change character set
CREATE DATABASE
CREATE SPFILE
ARCHIVELOG and RECOVERY
Includes the RESTRICTED SESSION privilege
Effectively, this system privilege allows a user to connect as user SYS.
SYSOPER
Perform STARTUP and SHUTDOWN operations
CREATE SPFILE
ALTER DATABASE OPEN/MOUNT/BACKUP
ARCHIVELOG and RECOVERY
Includes the RESTRICTED SESSION privilege
This privilege allows a user to perform basic operational tasks, but without the ability to look at user data