SQL injection is a technique that exploits a security 
vulnerability occurring in the database layer of an 
application. The vulnerability is present when user input 
is either incorrectly filtered for string literal escape 
characters embedded in SQL statements or user input is not 
strongly typed and thereby unexpectedly executed. It is in 
fact an instance of a more general class of vulnerabilities 
that can occur whenever one programming or scripting 
language is embedded inside another.