以sysdba进行访问oracle时提示如下信息：
[oracle@localhost ~]$ sqlplus / as sysdbaSQL*Plus: Release 19.0.0.0.0 - Production on Thu Dec 2 20:21:40 2021
Version 19.3.0.0.0Copyright (c) 1982, 2019, Oracle. All rights reserved.Connected to an idle instance.SQL> select * from dual;
select * from dual
*
ERROR at line 1:
ORA-01034: ORACLE not available
Process ID: 0
Session ID: 0 Serial number: 0
解决：
首先要保证启动监听：
[oracle@localhost ~]$ lsnrctl start然后启动instance：
SQL> startup;
这是再去查看数据库实例时就会是open的状态：
SQL> select status from v$instance;STATUS
------------------------
OPEN