1.登录情况：
[oracle@localhost ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.3.0 Production on Mon Jul 1 09:15:35 2019
Copyright (c) 1982, 2011, Oracle. All rights reserved.
Connected to an idle instance.
2.连接到空闲实例，可能是数据库监听未启动，或者数据库未启动，先启监听，再启数据库。
先查看监听状态
[oracle@localhost ~]$ lsnrctl status
LSNRCTL for Linux: Version 11.2.0.3.0 - Production on 01-JUL-2019 09:15:55
Copyright (c) 1991, 2011, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
TNS-12541: TNS:no listener
TNS-12560: TNS:protocol adapter error
TNS-00511: No listener
Linux Error: 111: Connection refused
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC1521)))
TNS-12541: TNS:no listener
TNS-12560: TNS:protocol adapter error
TNS-00511: No listener
Linux Error: 111: Connection refused
3.监听未启动，则先启动
[oracle@localhost ~]$ lsnrctl start
LSNRCTL for Linux: Version 11.2.0.3.0 - Production on 01-JUL-2019 09:16:01
Copyright (c) 1991, 2011, Oracle. All rights reserved.
Starting /u01/app/oracle/product/11.2.0/db_1/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 11.2.0.3.0 - Production
System parameter file is /u01/app/oracle/product/11.2.0/db_1/network/admin/listener.ora
Log messages written to /u01/app/oracle/diag/tnslsnr/localhost/listener/alert/log.xml
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=localhost)(PORT=1521)))
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC1521)))
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521)))
STATUS of the LISTENER
------------------------
Alias LISTENER
Version TNSLSNR for Linux: Version 11.2.0.3.0 - Production
Start Date 01-JUL-2019 09:16:02
Uptime 0 days 0 hr. 0 min. 0 sec
Trace Level off
Security ON: Local OS Authentication
SNMP OFF
Listener Parameter File /u01/app/oracle/product/11.2.0/db_1/network/admin/listener.ora
Listener Log File /u01/app/oracle/diag/tnslsnr/localhost/listener/alert/log.xml
Listening Endpoints Summary...
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=localhost)(PORT=1521)))
(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC1521)))
The listener supports no services
The command completed successfully.
4.登录数据库，若此处登录不上，先查看.bash_profile 中ORACLE_SID是否配置正确
[oracle@localhost ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.3.0 Production on Mon Jul 1 09:16:05 2019
Copyright (c) 1982, 2011, Oracle. All rights reserved.
Connected to an idle instance.
5.登陆后仍然是空闲实例，可能数据库未启动，启动数据库
SQL> startup
ORACLE instance started.
Total System Global Area 764121088 bytes
Fixed Size 2232272 bytes
Variable Size 494927920 bytes
Database Buffers 264241152 bytes
Redo Buffers 2719744 bytes
Database mounted.
Database opened.
6.启动成功，查询操作
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE 11.2.0.3.0 Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
7.结束。