I am using Ubuntu 11.10 and I want to learn SQL and PL/SQL. Can someone please advice me the most suitable database that I should install?
PL/SQL is a procedural language used in Oracle: http://en.wikipedia.org/wiki/PL/SQL
So, if you want to learn PL/SQL, then Oracle is your only choice.
If you want to learn SQL in general, then PostgreSQL, MySQL or even SQLite are quite suitable for the task.