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In this blog i have explained you how to resolve ORA-01442 .

ORA-01034: ORACLE not available

Cause: Oracle was not started up. Possible causes include the following:

- The SGA requires more space than was allocated for it.

- The operating-system variable pointing to the instance is improperly defined.

Action: Refer to accompanying messages for possible causes and correct the problem mentioned in the other messages. If Oracle has been initialized, then on some operating systems, verify that Oracle was linked correctly. See the platform specific Oracle documentation.

There could be a many reasons for ORA-01034, some of them are as following

1) Oracle Instance is not started.

2) ORACLE_SID or ORACLE_HOME or both are not properly defined.

3) Size of SGA is smaller than an Oracle Instance requires.

4) Remote user might face it due to improper configuration of Listener.

5) Refer to accompanying messages for possible causes and correct the problem.

One of the most common reason for ORA-01034 is Oracle Instance is down. Lets reproduce ORA-01034 by shutting down an Oracle instance and try reconnecting it.

SQL*Plus: Release 11.2.0.3.0 Production on Fri May 23 08:57:00 2014
Copyright (c) 1982, 2011, Oracle.  All rights reserved.
Connected to:
Oracle Database 11g Release 11.2.0.3.0 - Production

SQL> shutdown abort
ORACLE instance shut down.
SQL> exit

C:\Users\nimish.garg>sqlplus scott/tiger
SQL*Plus: Release 11.2.0.3.0 Production on Fri May 23 08:57:34 2014
Copyright (c) 1982, 2011, Oracle.  All rights reserved.

ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Process ID: 0
Session ID: 0 Serial number: 0

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