Oracle - Database | Troubleshooting

Issue / Error Message: The Network Adapter could not establish the connection
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Scenario: Database connection is failing. You either see this error in application log that is not starting because it is not able to connect to database OR you are using some tools/IDE to connect to database and see the above error

Cause: Either the database listener is not started or you are providing incorrect connectivity details

Solution: Start listener or ask your DBA to do so OR verify if database connectivity details like hostname, port, SID/service name are correct.