SQL injection is a serious vulnerability for online applications that occurs when untrusted data is added to a database query, allowing attackers to steal valuable data or bypass authentication. To prevent SQL injection attacks, developers can follow best practices such as not relying on client-side input validation, using a database user with restricted privileges, and using prepared statements and query parameterization. Scanning code for SQL injection vulnerabilities and using an ORM layer can also help prevent these attacks. Additionally, performing input validation, being careful with stored procedures, and avoiding blocklisting are important measures to take. By following these best practices, developers can significantly reduce the risk of SQL injection attacks in their applications.