I have a GUI system that is connected to the derby database. I want to check whether a user has taken any two books if so an error message comes up saying cannot borrow any more books anything less than that is okay.
Actually, a common way to check is to simply attempt to try and add the record and catch the Exception if it fails. Unless people are doing a lot of duplicates, it's less overhead than reading, then writing.
For such a mechanism to work, however, at least one column in the table has to have been defined as UNIQUE. If you've defined a PRIMARY KEY column, that will be the case, as PRIMARY KEYs are inherently unique.
Neither this solution nor Tom Moores's solution are Derby-specific. They apply to any SQL database.
Loudly announcing something is true and finding out you're wrong makes you feel foolish.
Finding out you're wrong and refusing to admit it makes you LOOK foolish.