SQL> select open_mode from v$database; 
OPEN_MODE
--------------------
MOUNTED

SQL> alter system set db_unique_name='testre' scope=spfile sid='*' ;
System altered.

SQL> shut immediate 
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.

SQL> startup mount 
ORACLE instance started.
Total System Global Area 1.0689E+10 bytes
Fixed Size                  2262656 bytes
Variable Size            3724544384 bytes
Database Buffers         6878658560 bytes
Redo Buffers               84008960 bytes
Database mounted.
SQL> show parameter db_unique_name 
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_unique_name                       string      testre
SQL> 

SQL> alter system set db_unique_name=testre scope=spfile sid='*' ;       ---- 注意，没有单引号，或双引号  
System altered.

SQL> shut immediate 
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.

SQL> startup mount 
 
SQL> show parameter db_unique_name  

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_unique_name                       string      TESTRE