I needed to understand if there's any system which let me create my very own tables and execute query on individuals tables? I would like these because I haven't any database placed on my office computer and I wish to practice queries during my spare time.
[The next answer largely assumes you're using Home windows.]
For any extremely swift-n-dirty sandbox, try SQLite. It arrives with sqlite3.exe (begin to see the Precompiled Binaries For Home windows section), which allows you create tables and run queries them over in the command-line.
SQLite's dialect of SQL is a little strange because of its dynamic typing model, so you may have more mileage from setting up a far more conventional database like PostgreSQL, although it is much more involved than SQLite's "installation", which comes down to removing the .exe file and adhering it somewhere in your path.