How do I prevent SQL Injection attacks?
Firewalls and similar intrusion detection mechanisms provide little defense against full-scale web attacks. Since your website needs to be public, security mechanisms will allow public web traffic to communicate with your databases servers through web applications. Isn’t this what they have been designed to do?
Patching your servers, databases, programming languages and operating systems is critical but will in no way the best way to prevent SQL Injection Attacks.

How to Check for and Prevent SQL Injection Attacks?
Download Acunetix Web Vulnerabiltiy Scanner and find out if your website is vulnerable!
http://www.acunetix.com/vulnerability-scanner/download/