I use MySQL for most of my research and “hobbyist” (vice enterprise) database needs. There are a few pros and cons for using MySQL, summarized in the following links:

• Eight reasons not to use MySQL
• Five reasons to use MySQL

SQL Injection is a common technique to hack a website, inserting malicious text in forms to take access the backend database in ways not intended by the web developer.

The following link gives a list of free SQL Injection scanners. 

http://www.security-hacks.com/2007/05/18/top-15-free-sql-injection-scanners