SQL> alter database mount;
alter database mount
*
第 1 行出现错误:
ORA-00750: database has been previously mounted and dismounted

SQL>

[oracle@localhost ~]$ oerr ora 750
00750, 00000, "database has been previously mounted and dismounted"
// *Cause:  The instance has already mounted and dismounted the database,
//          which is only allowed once in its lifetime.
// *Action: Shut down the database.
[oracle@localhost ~]$

数据库“一生中”只能装载卸载一次，想必ORACLE的设计者是出于某种特殊的考虑吧。