ORA-12560 "TNS protocol adapter error" (Tns: Protocol Adapter Error Ora-12560 Connecting Via Sqlplus)

Cause:  The ORACLE_SID is not set in the environment

Solution:

To implement the solution, please execute the following steps (for Multiple Oracle Homes Or One oracle Home):
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ORACLE_SID variables should only be set in the appropriate registry key that corresponds to the software installation. In this case it should be set back to Original (old) instance name for taking it as default ORACLE_SID.

For 8i and 9i, this would be:
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HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\HOME(n)


For 10g and 11g, this would be:
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HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\ORACLE\KEY_

OR

You may execute the following steps (If only one Oracle Home installed ) to set variables at system level :
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1. Go to: My Computer >> right click >> Properties >> Advanced >> Env variables >> System

2. Add new variable

3. Enter ORACLE_SID as the variable name

4. Enter the database sid as the value