Oracle XE "Database" vs "Scheme"

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In Oracle, a database means a collection of data files and control files stored on disk. You can have only one database with Oracle XE. A database is mounted by a Oracle instance, which means all the background daemons and programs that you can see with ps.

You probably want a schema, which is what other DBMS products (e.g. MySQL) often refer to as a "database". IIRC there was no limit on the number of schemas in Oracle XE. You can create those from the APEX UI that comes with XE, or from command line (with CREATE USER -- again, confusingly, a userand schema are largely synonymous in this case).

I recommend reading the Concepts guide from Oracle documentation, most of this basic stuff (e.g. how things are called in Oracle-lingo) is covered there.