Quality is delighting customers
SQL injection is one of the oldest but still most common technique of hacking any application, wherein we fire SQL queries to the application to fetch some information which otherwise we are not authorized to access.
Whereas, security testing is broad concept that includes testing every aspect of system security. I am attaching a very detailed OWASP security testing guide.
Hope that help! But.... its a specialist job.
SQL Injection is one of the types of Security Testing. In security testing we test the application from each & every aspect.