Attackers use NULL values to probe the database because NULL maps to almost any data type, allowing them to figure out the exact number of columns the database is expecting without triggering a data-type error. Attackers use NULL values to probe the database

Explain how attackers systematically increase the number of NULL values (e.g., SELECT NULL, NULL, NULL ) until the application stops throwing a "500 Internal Server Error" and returns a valid page. {KEYWORD}') UNION ALL SELECT NULL#

This is the gold standard for stopping SQLi. It ensures the database treats user input strictly as data, never as executable code.

This operator combines the results of the original query with a new, injected query.