How to compare SQLite Column against a user string?

I am creating a JFrameApplet (In Java) with a log in (SQLite) but I am struggling to understand how to compare a wanted username, against a username that is already taken:

For Example: I want the username JoeBloggs, but it is taken, how do I compare a wanted username against one already made.

I have an SQLite users.db and the field is USERNAME.

Thank you for any assistance.