There is no custom made sample database, try to just install a database, use documentation and recommemded books to get a hint of the database design /size dependent.
Try all three possibilities small, medium and large.
create 1 via svrmgrl and the other thru dbassist to see evaluate the differences of database creation.
WEll if you're using oracle documentation, then familiarity with the examples can be exersied by running the scott or utlsample.sql to create the basisc tables.
Just practice on your machine and do everything to make it worse for you. by the book(certain standard failures) and unexpected failures.
That's the only way you'll be able to assess your competence in oracle databases.
I'm a JOLE(JavaOracleLinuxEnthusiast)
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