This error is because the connect identifier given, wrongalias, cannot be resolved
into database connection details by the TNS (Transparent Network Substrate—not an
acronym particularly worth remembering) layer of Oracle Net. The name resolution
method to be used and its configuration is a matter for the database administrator. In
this case, the error is obvious: the user entered the wrong connect identifier.
The second connect attempt gives the correct identifier, orcl.
This fails with
ORA-12541: TNS:no listener
This indicates that the connect identifier has resolved correctly into the address
of a database listener, but that the listener is not actually running. Note that another
possibility would be that the address resolution is faulty and is sending SQL*Plus
to the wrong address. Following this error, the user should contact the database
administrator and ask him or her to start the listener. Then try again.
The third connect request fails with
ORA-12514: TNS:listener does not currently know of service
requested in connect descriptor
This error is generated by the database listener. SQL*Plus has found the listener
with no problems, but the listener cannot make the onward connection to the database
service. The most likely reason for this is that the database instance has not been
started, so the user should ask the database administrator to start it and then try again.