Oracle SQL. The first difficulties that users overcome

How to find out the Oracle version

The Oracle version can be found by requesting select version from v$instance; in SQL Plus

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"No TNS entries found" Error message

In order to remove this message set TNS_ADMIN environment variable to the path where the file tnsnames.ora is located.
TNS stands for Transparent Network Substrate is a proprietary Oracle computer-networking technology, supports homogeneous peer-to-peer connectivity on top of other networking technologies such as TCP/IP, SDP and named pipes © Wikipedia.

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"No valid Oracle clients found"

This error occurs when one try to connect an Oracle database, through a Client like Toad for Oracle. To solve add the folder where Oracle client is located to the Path variable.

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Starting with Oracle 10g, along with the exp, imp utilities, appeared two new Data Pump utilities: expdp and impdp respectively.
They are also located in the bin directory of the Oracle home folder.
The old export and import utilities are client-side oriented, while the new ones are server-side oriented. Although the old and new utilities are similar in usage, the results are completely different. This means that a dump exported with exp cannot be imported with impdp.

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Default directory for the dmp file

If the directory is not specified, Oracle tries to seek the dmp file in the default directory, which is something like that
«drive letter»:\app\«user name»\admin\oratest\dpdump or
«drive letter»:\app\«user name»\product\21c\admin\xe\dpdump in newer versions of Oracle

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How to view already available directories for Data Pump

This request select directory_name, directory_path from dba_directories; in SQL Plus shows available directories.