What is second level SQL Injection

Question

What is all about the second level SQL Injection.. This is with reference to the question Use of parameters for mysql_query.. and a part of one of the answers had this term

Answer 1

If you just never assume ANY data is "safe" to put into a query, then you shouldn't have any problems with SQL Injection. Even if him/herself hands you piece of data and tells you to insert it into SQL query... you should still verify it! xD

It's a good idea to try to always use Parameterized Statements when executing queries. That way the database driver itself is responsible for escaping potentially harmful data, vastly reducing the risk of any type of SQL injection.