How can you make your database more hacker-proof?

Helpful books: (Security Engineering by Ross Anderson) (Computer and information security handbook by John R Vacca), (JOE Valacich and Christoph information system today, managing in the Digital World), (Computer Security by Micheal T Goodrich Roberto Tamassia) How can you make your database more hacker-proof? How do you ensure the security of an organizations’ distributed databases? (Security Databases)