Hackers can hack any databases. It is more important to secure your database by taking necessary precautions on server-side. I think MySQL is best and easy to manage database solution.
It isn't simple solution, you need to take certain measures. As is permissions for database users, setting strong passwords, as far as possible to use only local connection to the database and etc.
Your code that uses SQL databases will be vulnerable to SQL injection and all the hacks you see is from poorly written PHP/SQL code that allows an injection to happen for theft of data / information.
All of the known databases (Oracle, MySQL, PostgreSQL, etc.) are very secure but the security issues arise when you write/use insecure code or deploy your application on an unsecured environment.